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.

--------
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

#165 more accurate asyncio.CancelledError handling