For WCFS to be efficient it will have to carefully preserve OS cache on
file invalidations. As preparatory step establish infrastructure for
verifying state of OS file cache and start asserting on OS cache state
in a couple of places.

See comments added to tFile constructor that describe how OS cache state
verification is setup.

Some preliminary history:

kirr/wendelin.core@8293025b    X Thoughts on how to avoid readahead touching pages of neighbour block
kirr/wendelin.core@3054e4a3    X not touching neighbour block works via setting MADV_RANDOM in last 1/4 of every block
kirr/wendelin.core@18362227    X #5 access still triggers read to #4 ?
kirr/wendelin.core@17dbf94e    X Provide mlock2 fallback for Ubuntu
kirr/wendelin.core@d134c0b9    X wcfs: test: try to live with only hard memlock limit adjusted
kirr/wendelin.core@c2423296    X Fix mlock2 build on Debian 8

Provide filesystem view of in-ZODB ZBigFiles, but do not implement support for
invalidations nor isolation protocol yet. In particular, because ZODB
invalidations are not yet handled, the filesystem does not update its data in
accordance with ZODB updates, and instead provides stale data view that
corresponds to the state of ZODB at the time when wcfs was mounted.

The main parts of this patch are:

- wcfs/wcfs.go is filesystem implementation itself together with overview.
- wcfs/__init__.py is python wrapper to spawn and interoperate with that filesystem.
- wcfs/wcfs_test.py is tests.

Some preliminary history:

kirr/wendelin.core@fe7efb94    X start of wcfs
kirr/wendelin.core@878b2787    X draft loading
kirr/wendelin.core@d58c71e8    X don't overalign end by 1 blksize if end is already aligned
kirr/wendelin.core@29c9f13d    X readBlk: Fix thinko in already case
kirr/wendelin.core@59552328    X wcfs: Care to disable OS polling on us
kirr/wendelin.core@c00d94c7    X workaround lack of exception chaining on Python2 with xdefer
kirr/wendelin.core@0398e23d    X bytearray turned out to be copying data
kirr/wendelin.core@7a837040    X print wcfs.py py-level traceback on SIGBUS (e.g. wcfs.go aborting due to bug/panic)
kirr/wendelin.core@661b871f    X make sure tests don't get stuck even if wcfs gets killed -9 ...
kirr/wendelin.core@2c043d29    X More effort to unmount failed wcfs.go
kirr/wendelin.core@1ccc4478    X Use `with gil` + regular py code instead of PyGILState_Ensure/PyGILState_Release/PyRun_SimpleString
kirr/wendelin.core@5dc9c791    X wcfs: Kill xdefer
kirr/wendelin.core@91e9eba8    X wcfs: test: Register tFile to tDB early
kirr/wendelin.core@a7138fef    X wcfs: mkdir /tmp/wcfs with sticky bit
kirr/wendelin.core@1eec76d0    X wcfs: try to set sticky for /tmp/wcfs even if the directory already exists
kirr/wendelin.core@c2c35851    X wcfs: tests: Factor-out waiting for a general condition to become true into waitfor
kirr/wendelin.core@78f36993    X wcfs: test: Fix thinko in getting /sys/fs/fuse/connection/<X> for wcfs
kirr/wendelin.core@bc9eb16f    X wcfs: tests: Don't use testmntpt everywhere
kirr/wendelin.core@6dec74e7    X wcfs: tests: Split tDB into -> tDB + tWCFS
kirr/wendelin.core@3a6bd764    X wcfs: tests: Run `fusermount -u` the second time if we had to kill wcfs
kirr/wendelin.core@112720f3    X wcfs: tests: Print which files are still opened on wcfs if `fusermount -u` fails
kirr/wendelin.core@bb40185b    X wcfs: Take $WENDELIN_CORE_WCFS_OPTIONS into account not only from under join
kirr/wendelin.core@03a9ef33    X wcfs: Remove credentials from zurl when computing wcfs mountpoint
kirr/wendelin.core@68ee5bdc    X wcfs: lsof tweaks
kirr/wendelin.core@21671879    X wcfs: Teach entrypoint frontend to handle subcommands: serve, status, stop
kirr/wendelin.core@b0642b80    X wcfs: Switch mountpoints from /tmp/wcfs/* to /dev/shm/*
kirr/wendelin.core@b0ca031f    X wcfs: Teach join/serve to start successfully even after unclean wcfs shutdown
kirr/wendelin.core@5bfa8cf8    X wcfs: Add start to spawn a Server that can be later stopped  (draft)
kirr/wendelin.core@5fcec261    X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
kirr/wendelin.core@669d7a20    fixup! X wcfs: Run fusermount and friends with /bin:/usr/bin always on path
kirr/wendelin.core@6b22f8c4    X wcfs: Teach start to start successfully even after unclean wcfs shutdown
kirr/wendelin.core@15389db0    X wcfs: Tune _fuse_unmount to include `fusermount -u` error message into raised exception
kirr/wendelin.core@153c002a    X wcfs: _fuse_unmount: Try first `kill -TERM` before `kill -QUIT` wcfs
kirr/wendelin.core@3244f3a6    X wcfs: lsof +D misbehaves - don't use it
kirr/wendelin.core@a126e709    X wcfs: Put client log into its own logger
kirr/wendelin.core@ac303d1e    X wcfs: tests: -v  ->  show only wcfs.py logs verbosely
kirr/wendelin.core@d671a9e9    X wcfs: Give more time to stop wcfs server

Add functionality to load objects from ZODB as saved by py wendelin.core.
Mostly straightforward code.
The main part is in zblk.go .

Contrary to python implementation, go can load ZBlk1's subobjects in
parallel, which, given scalable ZODB storage, can be significantly
faster compared to serially loading all ZData subobjects as py code
does.

TODO test wrt data saved by Python3.

Some preliminary history:

kirr/wendelin.core@878b2787 X draft loading
kirr/wendelin.core@bf9a7405 X No longer rely on ZODB cache invariant for invalidations
kirr/wendelin.core@0d62b05e X Adjust to btree.VGet & friends signature change to include keycov in visit callback
kirr/wendelin.core@b74dda09 X Start switching Track from Track(key) to Track(keycov)

Add initial stub for WCFS program and tests.
WCFS functionality will be added step-by-step in follow-up commits.

Some preliminary history:

0ae88a32       X .nxdtest: Verify Go bits with GOMAXPROCS=1,2,`nproc`
23528eb4       X wcfs: make it to use go modules for dependencies

In 6637d216 (lib/zodb: Add zstor_2zurl - way to convert a ZODB storage
into URL to access it) we added zstor_2zurl function to convert a ZODB
storage client object into an URL to access the storage. At that time
the function knew how to understand FileStorage only. Let's add support
for other storages that WCFS will need to support now.

NEO URI scheme matches the one currently used on ZODB/go side. It
semantically needs nexedi/neoppod!18
to be also applied to NEO/py side, but we do not care for now that that
patch is not merged (yet, or forever) because extracted ZURL is used
only with WCFS which uses NEO/go.

NEO support also depends on custom patch to remember SSL credentials on
NEO Client:

kirr/neo@a2f192cb

Some preliminary history:

kirr/wendelin.core@5cb39463    fixup! X wcfs/zeo started to work locally
kirr/wendelin.core@1cf3b228    X zstor_2zurl += NEO
kirr/wendelin.core@7f8fa32a    X lib/zodb: zstor_2zurl += NEO/SSL support
kirr/wendelin.core@e26524df    X wcfs, lib/zodb: DemoStorage support

Upcoming libwcfs (C++ part of WCFS client) will need to use virtmem code
and link to libvirtmem.

Manaully, because there is no automatic dependency tracking in
setuptools...

Dependency tracking is needed to avoid miscompilation after incremental
update under SlapOS/buildout/testnode/... when e.g. only .h was changed.

This is similar to e870781d (Top-level in-tree import redirector) but
for upcoming pyx modules.

Soon we are going to split virtmem code into its own DSO to which
bigfile extension will link. As plain setuptools does not support such
dynamic linking, we are going to use setuptools_dso instead. But more:
some of our upcoming extensions and DSOs will need to use Cython and C++
parts of Pygolang. Prepare that and use Extensions and DSO from
golang.pyx.build to support that right from the start.

Currently we have only one extension wendelin.bigfile._bigfile, but we
are going to add more both python extensions and non-python DSOs. Start
preparing to that by factoring-out common code.

lib/tests/testprog/zloadrace.py:90:1 'ZODB.FileStorage.FileStorage' imported but unused

This amends commit c37a989d.

Do what we can do without gdb and then tail to regular segmentation
fault. With core file gdb can still be used, but it is handy if we
already can get traceback of the crash into the log automatically.

TODO better use https://github.com/ianlancetaylor/libbacktrace because
backtrace_symbols often does not provide symbolic information.

We do not do this now because libbacktrace is not always automatically
installed.

This makes sure that those programs are always built afresh instead
being stuck at outdated build. This is needed because corresponding test .c
file includes many other .c files and we don't implement dependency tracking.

Else, e.g. after a failing test, that closed its storage and DB, but not
all Connections, another test, just by starting new transaction, would
invoke synchronization on that unclosed connection, which will try to
access closed storage and likely fail.

Fixes e.g. https://nexedijs.erp5.net/#/test_result_module/20210401-31B27B3D/5

Crash scenariou is the same as described in 5a5ed2c7 (tests: Force-close
ZODB connections in teardown, that testing code forgot to explicitly
close). Only now we try to isolate tests from each other not only for
different modules, but also for tests inside the same module.

The tests verify that there is no concurrency bugs around load,
Connection.open and invalidations. See e.g.

https://github.com/zopefoundation/ZODB/issues/290
https://github.com/zopefoundation/ZEO/issues/155

By including the tests into wendelin.core, we will have CI coverage for
all supported storages (FileStorage, ZEO, NEO), and for all supported
ZODB (currently ZODB4, ZODB4-wc2 and ZODB5).

ZEO5 is know to currently fail zloadrace.
However, even though ZODB#290 was fixed, ZEO5 turned out to also fail on zopenrace:

        def test_zodb_zopenrace():
            # exercises ZODB.Connection + particular storage implementation
    >       zopenrace.main()

    lib/tests/test_zodb.py:382:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
    <decorator-gen-1>:2: in main
        ???
    ../../tools/go/pygolang/golang/__init__.py:103: in _
        return f(*argv, **kw)
    lib/tests/testprog/zopenrace.py:115: in main
        test(zstor)
    <decorator-gen-2>:2: in test
        ???
    ../../tools/go/pygolang/golang/__init__.py:103: in _
        return f(*argv, **kw)
    lib/tests/testprog/zopenrace.py:201: in test
        wg.wait()
    golang/_sync.pyx:246: in golang._sync.PyWorkGroup.wait
        ???
    golang/_sync.pyx:226: in golang._sync.PyWorkGroup.go.pyrunf
        ???
    lib/tests/testprog/zopenrace.py:165: in T1
        t1()
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

        def t1():
            transaction.begin()
            zconn = db.open()

            root = zconn.root()
            obj1 = root['obj1']
            obj2 = root['obj2']

            # obj1 - reload it from zstor
            # obj2 - get it from zconn cache
            obj1._p_invalidate()

            # both objects must have the same values
            i1 = obj1.i
            i2 = obj2.i
            if i1 != i2:
    >           raise AssertionError("T1: obj1.i (%d)  !=  obj2.i (%d)" % (i1, i2))
    E           AssertionError: T1: obj1.i (3)  !=  obj2.i (2)

    lib/tests/testprog/zopenrace.py:156: AssertionError

Previously if an assert or something failed in spawned thread, the main
thread was usually spinning indefinitely = tests hang. -> Switch all
threading places to use sync.WorkGroup and this way if a thread fails,
all other threads are canceled and the exception is reported back to
wg.wait in main thread.

Since we start to go this route, NotifyChannel is reworked to fully use
channels instead of busy-waiting.

setuptools_dso 2 started to emit those autogenerated files.
See https://github.com/mdavidsaver/setuptools_dso/pull/15 for details.

NEO 1.9 was released in 2018 and is outdated by now. NEO 1.12 is
currently the latest NEO release.

After switching to ZODB >= 4 in the previous commit, we can safely
require zodbtools, because there is now no conflict in between
ZODB3/ZODB eggs.

It's been a while since last ZODB3 3.10.7 release in 2016 and the last
commit in upstream ZODB3 repository (3.10 branch) is from 2017. The
world switched since then to ZODB4 and to ZODB5 after that.

We were still requiring ZODB3, because ZODB3 3.11 egg was just a
dependency on newer ZODB, ZEO, BTrees and persistent; and this way we
could be supporting all ZODB3.10.x and  ZODB4 and ZODB5 via ZODB3.11.

However upcoming Wendelin.core 2, for its proper working, needs MVCC
semantic as implemented in ZODB5. This forces us, even for ZODB4, to
backport non-trivial bits from ZODB5 (see [1]). Maintaining ZODB3
support at this point becomes non-practical, because, to our knowledge,
there is no wendelin.core user that plans to continue using ZODB3
without switching to at least ZODB4 in the near future.

So goodbye ZODB3. Even though ZODB still stays with us, it gives a
feeling similar to [2], because in 2014, when I was myself learning
ZODB, it was through ZODB3 - still at the time when all ZODB bits were
living together in one place.

[1] ZODB!1
[2] https://lists.osuosl.org/pipermail/darcs-users/2008-September/014095.html

DB.close() does `del self.storage`.

https://github.com/zopefoundation/ZODB/blob/5.6.0-14-g0eae10cd0/src/ZODB/DB.py#L646

This way if DB was closed, but some conn(s) were not, it will crash in
teardown as e.g. below:

    _____________ ERROR at teardown of test_bigfile_zblk1_zdata_reuse ______________

        def teardown_module():
    >       testdb.teardown()

    bigfile/tests/test_filezodb.py:58:
    _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _

    self = <wendelin.lib.testing.TestDB_ZEO object at 0x7fb9c0216350>

        def teardown(self):
            # close connections that test code forgot to close
            for connref, tb in self.connv:
                conn = connref()
                if conn is None:
                    continue
                if not conn.opened:
                    continue    # still alive, but closed
                print("W: testdb: teardown: %s left not closed by test code"
                      "; opened by:\n%s" % (conn, tb), file=sys.stderr)

                db = conn.db()
    >           stor = db.storage
    E           AttributeError: 'DB' object has no attribute 'storage'

    lib/testing.py:217: AttributeError

The fix is simple - don't use db.storage at all, because it is not actually used in that code.

Since ZBigFile keeps references to fileh objects that are created
through it it forms a file <=> fileh cycle that is not collected without
cyclic GC:

https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L497
https://lab.nexedi.com/nexedi/wendelin.core/blob/v0.13-52-ga702d41/bigfile/file_zodb.py#L566-571

We did not noticed this leak until now because it is small, but with
upcoming wendelin.core 2 it is important to release a fileh, because
there is WCFS connection associated with fileh, and if fileh is not
released, that connection also stays alive, keeping on-WCFS resources
still being used, and preventing WCFS from being unmounted cleanly.

-> Add cyclic GC support to PyBigFile / PyBigFileH

NOTE: we still don't allow PyVMA <=> PyBigFileH cycles to be collected,
because fileh_close called from fileh.__del__ asserts that there are no
live mappings left. See added comments for details. There is no
known practical need to use such cycles, so this should be ok.

See also other patches on cyclic GC topic:

- 450ad804 (bigarray: ArrayRef support for BigArray)  // adds cyclic GC support for PyVMA
- d97641d2 (bigfile/py: Properly untrack PyVMA from GC before dealloc)

/proposed-for-review-on nexedi/wendelin.core!12

The logic in pyvma_traverse and pyvma_clear needs to be synchronized
with PyVMA deallocation. In the next patche we'll be amending this
logic, and it will help a reader to keep all those functions together.

For the reference: PyVMA support for cyclic GC was introduced in
450ad804 (bigarray: ArrayRef support for BigArray). See also d97641d2
(bigfile/py: Properly untrack PyVMA from GC before dealloc).

/proposed-for-review-on nexedi/wendelin.core!12

Otherwise when /bin/sh is dash it fails with

    t/tfault-run: 35: test: on_pagefault: unexpected operator

Otherwise, if previous test.fault failed, tfault-run fails to start, e.g.

    >>> test.fault
    $ make test.fault # MAKEFLAGS=-j1
    x86_64-linux-gnu-gcc -pthread -g -Wall -D_GNU_SOURCE -std=gnu99 -fplan9-extensions -Wno-declaration-after-statement -Wno-error=declaration-after-statement  -Iinclude -I3rdparty/ccan -I3rdparty/include   bigfile/tests/tfault.c lib/bug.c lib/utils.c 3rdparty/ccan/ccan/tap/tap.c  -o bigfile/tests/tfault.t
    t/tfault-run bigfile/tests/tfault.t faultr on_pagefault
    mkdir: cannot create directory ‘t/tfault-run.faultr’: File exists
    Makefile:186: recipe for target 'faultr.tfault' failed
    make: *** [faultr.tfault] Error 1
    rm bigfile/tests/tfault.t
    error   test.fault      0.433s  # 1t 1e 0f 0s

Nxdtest[1] is tox-like tool to run tests under Nexedi testing
infrastructure. See [2] for details.

[1] https://lab.nexedi.com/nexedi/nxdtest
[2] slapos!839

We need the following patch of mine:

   http://git.ozlabs.org/?p=ccan;a=commitdiff;h=b97c7f0841f5173a07a2571f2c99f944d8405a90

It is hard for people to understand current wording, so let's expand
zconn_at description to additionally explain what it is providing with
second set of words, which, hopefully, lowers potential ambiguity a bit.

/reported-by @jwolf083

Pygolang egg provides "golang" python package, not "pygolang".
Bug introduced in 5c8340d2 (*: Use defer for dbclose & friends).

In PEP517 mode setup.py is sourced - not executed - and the build fails
with ImportError like this:

    Preparing wheel metadata ... error
    ERROR: Command errored out with exit status 1:
     command: /home/kirr/src/wendelin/venv/z-dev/bin/python2 /home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py prepare_metadata_for_build_wheel /tmp/tmp2F3aEs
         cwd: /home/kirr/src/wendelin/wendelin.core
    Complete output (53 lines):
    running dist_info
    creating /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info
    writing requirements to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/requires.txt
    writing /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/PKG-INFO
    writing top-level names to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/top_level.txt
    writing dependency_links to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/dependency_links.txt
    writing entry points to /tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/entry_points.txt
    writing manifest file '/tmp/pip-modern-metadata-sPiqUt/wendelin.core.egg-info/SOURCES.txt'
    package init file '__init__.py' not found (or not a regular file)
    Traceback (most recent call last):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 257, in <module>
        main()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 240, in main
        json_out['return_val'] = hook(**hook_input['kwargs'])
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pip/_vendor/pep517/_in_process.py", line 110, in prepare_metadata_for_build_wheel
        return hook(metadata_directory, config_settings)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 155, in prepare_metadata_for_build_wheel
        self.run_setup()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 234, in run_setup
        self).run_setup(setup_script=setup_script)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/build_meta.py", line 141, in run_setup
        exec(compile(code, __file__, 'exec'), locals())
      File "setup.py", line 374, in <module>
        """.splitlines()]
      File "/home/kirr/src/tools/go/pygolang/golang/pyx/build.py", line 118, in setup
        setuptools_dso.setup(**kw)
      File "/home/kirr/src/tools/py/pypa/setuptools_dso/src/setuptools_dso/__init__.py", line 37, in setup
        _setup(**kws)
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/__init__.py", line 145, in setup
        return distutils.core.setup(**attrs)
      File "/usr/lib/python2.7/distutils/core.py", line 151, in setup
        dist.run_commands()
      File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
        self.run_command(cmd)
      File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
        cmd_obj.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/dist_info.py", line 31, in run
        egg_info.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 296, in run
        self.find_sources()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 303, in find_sources
        mm.run()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 534, in run
        self.add_defaults()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/egg_info.py", line 574, in add_defaults
        rcfiles = list(walk_revctrl())
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/setuptools/command/sdist.py", line 20, in walk_revctrl
        for item in ep.load()(dirname):
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2434, in load
        return self.resolve()
      File "/home/kirr/src/wendelin/venv/z-dev/local/lib/python2.7/site-packages/pkg_resources/__init__.py", line 2444, in resolve
        raise ImportError(str(exc))
    ImportError: 'module' object has no attribute 'git_lsfiles'

See comments added to register_as_entrypoint for explanation of what
happens.

Wendelin.core will soon switch to PEP517 mode (by adding pyproject.toml)
to build-require Cython, Pygolang and friends.

Wendelin.core 2 will need to spawn WCFS filesystem server that accesses
the same ZODB database as the program that spawns it. The database
argument passed to WCFS is passed in the form of URL[1,2].  Even though
zodburi provides way to convert an URL into ZODB storage instance, there
is currently no way for reverse operation - to convert ZODB storage
instance into URL to access it(*). So we have to build it by our own.

Provide zstor_2zurl stub that currently works for FileStorage only.
ZEO and NEO support is TODO.

In the future we might want to move this functionality into
zodbtools/py.

[1] https://lab.nexedi.com/nexedi/zodbtools/blob/a2e4dd23/zodbtools/help.py#L27-53
[2] https://lab.nexedi.com/kirr/neo/blob/3d909114/go/zodb/zodbtools/help.go#L25-51

(*) contrary to ZODB/go where this functionality is provided out of the box:
    https://godoc.org/lab.nexedi.com/kirr/neo/go/zodb#IStorage

Wendelin.core 2 will need to hook into when client ZODB.Connection
changes its database view and readjust WCFS-level client connection
accordingly.

ZODB.Connection can change its view on either connection reopen, or even
without reopen on start of new transaction.

This patch implements ZODB.Connection.onResyncCallback for ZODB5 only.

ZODB4 and ZODB3 support is TODO.

For wendelin.core v2 we need a way to know at which particular database
state application-level ZODB connection is viewing the database. Knowing
that state, WCFS client library will interact with WCFS filesystem server
and, in simple terms, request the server to provide data as of that
particular database state.

Contrary to ZODB/go[1] ZODB/py does not provide the functionality to
obtain DB state of connection view, so we have to build it ourselves.
Let us call the function that for a client ZODB connection returns
database state corresponding to its database view as zconn_at.

It is relatively easy to implement zconn_at for ZODB5, since ZODB5
adopted MVCC uniformly and this patch does just that. However even with
ZODB5 currently all released ZODB5 versions have race in
Connection.open() vs invalidations[2], and so the first ZODB5 release
with which zconn_at implemented here will work reliable should be
upcoming ZODB 5.5.2

It is TODO to implement zconn_at for ZODB4 and ZODB3, which organize
things differently.

Please note what would happen if zconn_at gives, even a bit, incorrect
answer: wcfs client will ask wcfs server to provide array data as of
different database state compared to current on-client ZODB connection.
This will result in that data accessed via ZBigArray will _not_
correspond to all other data accessed via regular ZODB mechanism.
It is, in other words, would be a data corruptions.

[1] https://godoc.org/lab.nexedi.com/kirr/neo/go/zodb#Connection
[2] https://github.com/zopefoundation/ZODB/issues/290

This will be needed in the following patches to know how to inject
zconn_at or zconn resync functionality into particular ZODB version.

- mention in comments that _ZBigFileH not only proxies changes from
  virtmem -> ZODB, but also the other way: virtmem <- ZODB.
- refresh comments, fix typo.

- Provide brief top-level overview + refresh loadblk/storeblk/release comments.
- Add `typedef struct bigfile_ops bigfile_ops` that we usually add for all structs.

It is valid to compare a Page and a VMA only if they belong to the same
fileh.

-> into vma_page_infilerange().

We will soon need to use this functionality from several places.

Start preparing vma early, not after the call to mem_valloc.
This codeflow will be more convenient when we add mmap-through-wcfs codepath.