/reviewed-by @d-maurer 
/reviewed-on https://github.com/zopefoundation/ZEO/pull/210

Since 2016 this mode was opt-in for a long time with the default being
single-threaded asyncio-based server implementation. We believe nobody
actually used multi-threaded ZEO5 server mode for real at all.

Multi-threaded server mode was deprecated this spring
(https://github.com/zopefoundation/ZEO/pull/190), and later found to
have concurrency bug that lead to data corruption
(https://github.com/zopefoundation/ZEO/issues/209).

Given deprecation status, instead of fixing those bugs let's finally
remove the multi-threaded server mode.

@d-maurer notes that --all on the command line prevents using `-a X` on
the tox command line, e.g. as in `tox -- -a 1` because `--all` takes
precedence over `-a`:

https://github.com/zopefoundation/ZEO/pull/210#pullrequestreview-1094352596

Work this around via `-a 1000` used inside tox -> then, any `-a X`
passed to tox on command line will take precedence over the default
`-a 1000`.

Note: Dieter suggested to use `-a 0` as the default, probably because
zope-testrunner documents that "level 0 runs all tests". But
unfortunately it is not true because `-a 0` disables tests on any level
>= 0:

https://github.com/zopefoundation/zope.testrunner/blob/eab00d6f/src/zope/testrunner/find.py#L471-L473

(z-dev) kirr@deca:~/src/wendelin/z/ZEO5$ zope-testrunner -uvvx -a 0 --test-path=src
Running tests at level 0
Total: 0 tests, 0 failures, 0 errors and 0 skipped in 0.000 seconds.

(z-dev) kirr@deca:~/src/wendelin/z/ZEO5$ zope-testrunner -uvvx -a 1 --test-path=src
Running tests at level 1
Running zope.testrunner.layer.UnitTests tests:
  Set up zope.testrunner.layer.UnitTests in 0.000 seconds.
  Running:
 testClientBasics (ZEO.asyncio.tests.ClientTests)
 test_ClientDisconnected_on_call_timeout (ZEO.asyncio.tests.ClientTests)
 test_bad_protocol (ZEO.asyncio.tests.ClientTests)
 test_bad_server_tid (ZEO.asyncio.tests.ClientTests)
...

Since https://github.com/zopefoundation/ZODB/commit/a94d63c552d6 (part
of https://github.com/zopefoundation/ZODB/pull/368) some ZODB tests are
marked as being long to help skipping them easily during development via
running tests only at level 1.

But by default we want to continue to run all tests.

-> Adjust tox to run tests at all levels explicitly, since
zope-testrunner's default is to run tests at level 1 only.

--------
kirr:

Those methods are not private to Protocol, because they are invoked e.g. from
other modules besides ZEO.asyncio.base - e.g. from ZEO.asyncio.server . Make
them public via dropping "_" prefix and naming them using more descriptive names.

Extracted from https://github.com/zopefoundation/ZEO/pull/195

--------
kirr:

Always use appropriate logger object set up in a module. It was only
ZEO/tests/ConnectionTests.py which was using logging directly even
though corresponding logger object was set up in that module.

Extracted from https://github.com/zopefoundation/ZEO/pull/195

--------
kirr:

Extract from https://github.com/zopefoundation/ZEO/pull/195 bits that
add documentation to existing code without changing semantic, and fix
typos.

The only things I added myself with further help from @d-maurer are:

- documentation for server_sync in ClientStorage;
- stub documentation for credentials in ClientStorage.

Even though we agree to deprecate credentials in favour of peer-to-peer
TLS, removing their support should go as a separate step.

For server_sync feature, that
https://github.com/zopefoundation/ZEO/pull/195 currently removes, we
actually do use it for correctness:

https://lab.nexedi.com/nexedi/erp5/blob/eaae74a082a0/product/ERP5Type/tests/custom_zodb.py#L175-179
erp5@c663257f
https://github.com/zopefoundation/ZODB/commit/9821696f584f

So document it with the intent to preserve it.

/reviewed-on https://github.com/zopefoundation/ZEO/pull/202

We can use stdlib random module and manually adjust randint to behave
exactly as before Python3, so that testdata are matched exactly as before.

Also drop unnecessary `random.seed(42)` in cache_run. It is not needed
there because random is not used in cache_run at all.

Based on work of @d-maurer in https://github.com/zopefoundation/ZEO/pull/195 .

Reduce ZEO testing time by dropping tests with some storages

Drop ZEO tests with storages `DemoStorage`, `FileStorage` and `HexFileStorage`; `FileStorage` is still tested in the variants with server side blobs and with shared blobs

Full linting with flake8

Co-authored-by: Dieter Maurer <d-maurer@users.noreply.github.com>

Co-authored-by: Dieter Maurer <d-maurer@users.noreply.github.com>

Configure with zope.meta.config.config-package

Python 3.10 support

tpc_vote/abort + log message enhancements

Remove testing for mtacceptor and dead Python versions

Python 3.10 distinguishes SSL contexts for server and client use - and SSL fails with the wrong context type