* master:
  go/*: Cosmetics
  go/xzodb: New package that provides zodb-related utilities shared by drivers  (draft)
  go/zodb/demo: New package that provides base+δ storages overlay
  go/zodb: Provide OpenDriver in addition to Open
  go/zodb/fs1: Report URL with file:// schema included
  go/zodb: Fix Open to detect lack of schema only by ":" instead of by "://"
  go/zodb: Fix IStorage implementation not to deadlock driver if watcher fails
  go/zodb/zeo: Fix it to provide "Close vs watchq" guaranty
  go/zodb/fs1: Fix it to provide "Close vs watchq" guaranty
  go/zodb: Require drivers to close watchq on Close
  go/internal/xtesting: Fix FatalIf to return a helper

For Load demo.Storage implementation is similar to DemoStorage in
ZODB/py with fixes "cherry-picked" from:

- https://github.com/zopefoundation/ZODB/issues/318
  (DemoStorage does not take whiteouts into account -> leading to data corruption)

- https://github.com/zopefoundation/ZODB/pull/323
  (loadAt + fix for the above issue)

For safety demo.Storage - contrary to DemoStorage/py - actually verifies
that for demo=base+δ δ comes strictly after base and that base remains
unchanged.

URI schema follows XRI Cross-references approach and is

	demo:(zurl_base)/(zurl_δ)

https://en.wikipedia.org/wiki/Extensible_Resource_Identifier provides
some related details and examples.

For ZODB/py corresponding pull-request for zodburi to support demo: URI
scheme has been made here: https://github.com/Pylons/zodburi/pull/29 .

Tests need:

- recent zodbtools with zodbrestore:

  https://lab.nexedi.com/nexedi/zodbtools/blob/129afa67/zodbtools/zodbrestore.py
  nexedi/zodbtools!19

- ZODB with support for DemoStorage.deleteObject
  https://github.com/zopefoundation/ZODB/pull/341

On Go side demo storage is needed for wendelin.core 2 because ERP5 uses
DemoStorage to run tests.

This is low-level API to open IStorageDriver instead of IStorage.
Demo storage will need this.

Maybe it would be a good idea to move drivers-related functionality into
separate package in the future.

In ZODB/go when there is no schema in zurl, open automatically prepends
file:// . However filename itself could contain ":" and so generally
speaking it is incorrect to return URL without file:// schema prepended
to file name.

Another reason to always use fully-constructed URLs with schema, is
interoperability with ZODB/py - there zodburi, when given zurl without
schema, does not make any assumption that it is of file:// kind and
rejects opening such URIs.

An URI schema is required to have ":" after it, but - even if frequently
used in practice - not //. We will soon introduce "demo:" URI scheme
that comes without //, so fix Open to detect schema presence just by ":"
and not to fixup "demo:..." url to "file://demo:..." automatically.

Before the patch if storage.watcher fails, storage.driver.Close is not
called, and so the driver will continue to send to .drvWatchq, but noone
is receiving from it.

a5dbb92b (go/zodb: Require drivers to close watchq on Close), provides
the guarantee that the driver will stop sending on drvWatchq right after
drv.Close call.

Provide guaranty that Close forces the driver to stop sending to watchq
and to close it. See a5dbb92b ("go/zodb: Require drivers to close watchq
on Close") for details.

Without the fix TestWatch fails with test timeout:

    panic: test timed out after 30s

    # Close waits for serve to stop
    goroutine 93 [semacquire]:
    sync.runtime_Semacquire(0xc000152170)
            /home/kirr/src/tools/go/go/src/runtime/sema.go:56 +0x45
    sync.(*WaitGroup).Wait(0xc000152168)
            /home/kirr/src/tools/go/go/src/sync/waitgroup.go:130 +0x65
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).Close(0xc0001520f0, 0x1313, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:159 +0x47
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).Close(0xc000313680, 0xc000107c78, 0x1)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:526 +0x2e
    lab.nexedi.com/kirr/neo/go/internal/xtesting.DrvTestWatch(0xc000082c00, 0xc0000aa2a0, 0x24, 0x6a4a38)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/internal/xtesting/xtesting.go:442 +0xdb5
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.TestWatch.func1(0xc000082c00, 0x6e3498, 0xc00009a380)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:270 +0x99
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2.1(0xc0000a4168, 0x16)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:207 +0xfb
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func1(0xc000082c00, 0xc00009c180)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:186 +0x129
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.withZEOSrv.func2(0xc000082c00)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo_test.go:199 +0x10e
    testing.tRunner(0xc000082c00, 0xc00009c160)
            /home/kirr/src/tools/go/go/src/testing/testing.go:1194 +0xef
    created by testing.(*T).Run
            /home/kirr/src/tools/go/go/src/testing/testing.go:1239 +0x2b3

    # serve is stuck in invalidateTransaction doing watchq<-
    goroutine 26 [chan send]:
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zeo).invalidateTransaction(0xc000313680, 0x6417e0, 0xc000323b60, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zeo.go:176 +0x373
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv1(0xc0001520f0, 0xc000393890, 0x0, 0x0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:225 +0x4b4
    lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).serveRecv(0xc0001520f0)
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:176 +0x8d
    created by lab.nexedi.com/kirr/neo/go/zodb/storage/zeo.(*zLink).start
            /home/kirr/src/neo/src/lab.nexedi.com/kirr/neo/go/zodb/storage/zeo/zrpc.go:99 +0xc8

Provide guaranty that Close forces the driver to stop sending to watchq
and to close it. See a5dbb92b ("go/zodb: Require drivers to close watchq
on Close") for details.

If we don't require drivers to stop sending to watchq after Close, there
could be many deadlock scenarios, for example:

- client called drv.Close(); no longer listens to watchq; driver is
  stuck sending to watchq, or

- client called drv.Close(), which itself waits for tasks spawned inside
  driver to complete, which are stuck sending to watchq because client
  no longer does <-watchq.

The change is similar in spirit to safety guaranty provided by high-level
Watcher where after DelWatch call it is guaranteed that there will be no
more sends to subscribed watchq (see c41c2907 "go/zodb: High-level
watching - initial draft") for details.

All drivers don't provide requested guarantee yet.
We'll be fixing them one-by-one in followup commits.

Else, on an error, it is the lineno of `t.Fatal(err)` inside FatalIf
that is printed, not the line number inside user test.

ZEO and NEO are not yet adjusted to new "watchq vs Close" requirement.

For https://github.com/DataDog/czlib/pull/5