* master: (31 commits)
  go/zodb/zeo: Make "no serial on load after object deleted" to be a known bug
  go/zodb/zeo: Cosmetics
  go/zodb/zeo: Load: Fix thinko when rejecting result
  go/zodb/zeo: Add support for invalidations
  go/zodb/zeo: Teach zlink to serve incoming notification and requests
  go/zodb/zeo: MsgPack support
  go/zodb/zeo: Accept both int|bool for pickled message flags
  go/zodb/zeo: Factor-out tuple handling into encoding
  go/zodb/zeo: Factor-out string|bytes handling into encoding
  go/zodb/zeo: proto: Use enc.<X> and enc.as<X> to encode/decode type X
  go/zodb/zeo: Introduce notion of encoding
  go/zodb/zeo: Factor-out code to prepare pkt from message into pktEncode
  go/zodb/zeo: Start moving things related to protocol and messages encoding into proto.go
  go/zodb/zeo: Shutdown zlink on first error
  go/zodb/zeo: Use zlink as term for link in between peers
  go/zodb/zeo: Don't avoid defer
  go/zodb/zeo: errgroup -> xsync.WorkGroup
  go/zodb/zeo: Initial tests
  go/zodb/zeo: Fix thinko in openByURL error context
  go/internal/xtesting: LoadDBHistory
  ...

This turns zeo tests to pass.
The bug is reported upstream here: https://github.com/zopefoundation/ZODB/issues/318

This turns zeo tests to pass.
The bug is reported upstream here: https://github.com/zopefoundation/ZODB/issues/318

If xres fails to convert to tuple, we should error about xres, not res
which is always tuple(nil).

Receive invalidation from server and send corresponding events to watchq.
Care to send only events with tid > at0 that we initially returned when opening.

Tests pass, but they need https://github.com/zopefoundation/ZEO/pull/160

This will be used in the next patch to handle invalidateTransaction
messages.

ZEO5 adds way for messages to be encoded via either pickles or MessagePack.
However until now we were always using pickles.
Let's add msgpack support to be able to e.g. use wire encoding that
server prefers.

MsgPack support is almost fully localized in encoding.
We use tinylib/msgp runtime routines to decode/encode msg fields with known types,
and shamaton/msgpack to decode/encode msg.arg, which is interface{},
because msgp does not generally work for arbitrary reflections.

For msgpack=true, tests state is the same as with pickles: handshake
works, but load fails when verifying that Load returns correct error for
deleted object:

    TestLoad/py/msgpack=false: xtesting.go:272: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
        have: /tmp/zeo170183943/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: no such object
        want: /tmp/zeo170183943/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
    TestLoad/py/msgpack=false: xtesting.go:272: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
        have: /tmp/zeo170183943/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: no such object
        want: /tmp/zeo170183943/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
    TestLoad/py/msgpack=false: xtesting.go:290: load 7fffffffffffffff:0000000000000007: returned err unexpected:
        have: /tmp/zeo170183943/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: no such object
        want: /tmp/zeo170183943/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
    TestLoad/py/msgpack=false: xtesting.go:290: load 7fffffffffffffff:0000000000000006: returned err unexpected:
        have: /tmp/zeo170183943/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: no such object
        want: /tmp/zeo170183943/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6

    TestLoad/py/msgpack=true: xtesting.go:272: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
        have: /tmp/zeo247652538/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: no such object
        want: /tmp/zeo247652538/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
    TestLoad/py/msgpack=true: xtesting.go:272: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
        have: /tmp/zeo247652538/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: no such object
        want: /tmp/zeo247652538/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
    TestLoad/py/msgpack=true: xtesting.go:290: load 7fffffffffffffff:0000000000000007: returned err unexpected:
        have: /tmp/zeo247652538/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: no such object
        want: /tmp/zeo247652538/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
    TestLoad/py/msgpack=true: xtesting.go:290: load 7fffffffffffffff:0000000000000006: returned err unexpected:
        have: /tmp/zeo247652538/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: no such object
        want: /tmp/zeo247652538/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6

This is due to https://github.com/zopefoundation/ZODB/issues/318

It is already documented in pktDecodeZ comment that flags is int|bool.
However until now we were decoding it only as int.

As is it was working, but it will fail when receiving e.g.
invalidateTransaction message, because ZEO/py server actually uses bool
when sending it:

    https://github.com/zopefoundation/ZEO/blob/5.2.1-20-gcb26281d/src/ZEO/asyncio/base.py#L139

Fix it.
This will be covered by watch tests, when watch support will be added in a followup patch.

Handle pickled lists as valid input when decoding tuples.

This is more uniform and will be followed by all data types in the next patch.
Here rename tid/oid pack/unpack routines correspondingly.
Add docstrings for X=tid|oid.

Keep information about which message encoding is used on the wire in
encoding type. Make pktDecode/pktEncode and data type conversion
utilities be methods of this type. For now there is only 'pickles'
encoding, but we'll soon introduce 'msgpack'.

Currently not everything related to pickles is localized in encoding -
we'll be moving more bits to encoding in the followup patches.

First step:

- move msg and msgFlags
- move pktDecode
- move functions to pack/unpack tid and oid

more to come.

The same behaviour as ZEO/py does.

Just renaming.

Starting from Go1.14 defer is no longer slow:
https://golang.org/doc/go1.14#runtime

See go123@515a6d14

As promised in b65f6d0f (go/zodb: Teach ZODB/go to access ZEO (draft))
let's start to add tests for ZEO/go client.

The client is tested against ZEO/py server.
Cover handshake and Load.

Load currently fails beause in ZODB/py POSKeyError does not allow to
distinguish whether it is no object at all or object exists and its data
was not found for tid_before.

    --- FAIL: TestLoad/py (0.22s)
        zeo_test.go:217: load 0285cbacc06d3a4c:0000000000000007: returned err unexpected:
            have: /tmp/zeo273044293/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: no such object
            want: /tmp/zeo273044293/1.fs.zeosock: load 0285cbacc06d3a4c:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c
        zeo_test.go:217: load 0285cbad858bf2e6:0000000000000006: returned err unexpected:
            have: /tmp/zeo273044293/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: no such object
            want: /tmp/zeo273044293/1.fs.zeosock: load 0285cbad858bf2e6:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        zeo_test.go:217: load 7fffffffffffffff:0000000000000006: returned err unexpected:
            have: /tmp/zeo273044293/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: no such object
            want: /tmp/zeo273044293/1.fs.zeosock: load 7fffffffffffffff:0000000000000006: 0000000000000006: object was deleted @0285cbad858bf2e6
        zeo_test.go:217: load 7fffffffffffffff:0000000000000007: returned err unexpected:
            have: /tmp/zeo273044293/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: no such object
            want: /tmp/zeo273044293/1.fs.zeosock: load 7fffffffffffffff:0000000000000007: 0000000000000007: object was deleted @0285cbacc06d3a4c

Problem reported upstream:

-> https://github.com/zopefoundation/ZODB/issues/318

No need to use trailing : suffix as xerr.Contextf automatically adds one.

This is utility to load whole content of a ZODB database.

It will be useful to load data to expect from e.g. dataok.fs and feed it
as txnvOk to DrvTestLoad when verifying another storage driver.

This is utility to create X function, similar to exc.Raisif, but that
would call t.Fatal instead.