* master:
  tests: Reset transaction synchronizers before every test run

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.

* master:
  lib/zodb: Add tests for critical ZODB properties that Wendelin.core 2 will depend on
  *: tests: don't hang on exception in non-main thread

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.