Remove testing for mtacceptor and dead Python versions

#165 more accurate asyncio.CancelledError handling

From https://github.com/zopefoundation/ZEO/pull/182#issuecomment-826630195 :

For `protocol.fut('lastTransaction')` vs `ClientDisconnected` in verify - I
would suggest to do the following instead: just move up the try/except that is
_already_ covering the code in verify a bit up - to also cover that
lastTransaction call. This try / except already catches `except Exception` and
does `self.register_failed(protocol, exc)` on a failure:

https://github.com/zopefoundation/ZEO/blob/1070adcbd17b83b9515c43b48ca38687783793e4/src/ZEO/asyncio/client.py#L488-L535

So if we just move its beginning `try` a bit up in here to also cover
`lastTransaction` call here:

https://github.com/zopefoundation/ZEO/blob/1070adcbd17b83b9515c43b48ca38687783793e4/src/ZEO/asyncio/client.py#L485-L488

it will work out automatically. It will work correctly because
`ClientDisconnected` is inheriting as

    ClientDisconnected -> ClientStorageError -> StorageError -> POSError -> Exception

It is also more logical to wrap the whole operation in verify in such a
try/except block, so that if _anything_ at _any_ point fails, we mark the whole
operation as failed.

hunt for the drop cache race condition

fix #150

Replaced 'cache-size-bytes' with 'cache-size' in the documentation

Fix typo client_label -> client-label for strings configuration files

let `lastTransaction` change only after invalidation processing