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

Add automated tests against the ZODB master branch

This fixes #142

Switch from Travis CI to GitHub Actions for testing

Currently loadBefore and prefetch spawn async protocol.load_before task,
and, after waking up on its completion, populate the cache with received
data. But in between the time when protocol.load_before handler is run
and the time when protocol.load_before caller wakes up, there is a
window in which event loop might be running some other code, including
the code that handles invalidateTransaction messages from the server.

This means that cache updates and cache invalidations can be processed on
the client not in the order that server sent it. And such difference in
the order can lead to data corruption if e.g server sent

	<- loadBefore oid serial=tid1 next_serial=None
	<- invalidateTransaction tid2 oid

and client processed it as

	invalidateTransaction tid2 oid
	cache.store(oid, tid1, next_serial=None)

because here the end effect is that invalidation for oid@tid2 is not
applied to the cache.

The fix is simple: perform cache updates right after loadBefore reply is
received.

Fixes: https://github.com/zopefoundation/ZEO/issues/155

The fix is based on analysis and initial patch by @jmuchemb:

https://github.com/zopefoundation/ZEO/issues/155#issuecomment-581046248

A tests corresponding to the fix is coming coming through

https://github.com/zopefoundation/ZEO/pull/170  and
https://github.com/zopefoundation/ZODB/pull/345

For the reference, my original ZEO-specific data corruption reproducer
is here:

https://github.com/zopefoundation/ZEO/issues/155#issuecomment-577602842
https://lab.nexedi.com/kirr/wendelin.core/blob/ecd0e7f0/zloadrace5.py

/cc @jamadden, @dataflake, @jimfulton
/reviewed-by @jmuchemb, @d-maurer
/reviewed-on https://github.com/zopefoundation/ZEO/pull/169

tests: Factor "Open another ClientStorage to the same server" into generic _new_storage_client() (#170)

This allows ZODB tests to recognize ZEO as client-server storage and so
make "load vs external invalidate" test added in
https://github.com/zopefoundation/ZODB/pull/345 to reproduce data
corruption problem reported at
https://github.com/zopefoundation/ZEO/issues/155.

For the reference: that problem should be fixed by
https://github.com/zopefoundation/ZEO/pull/169.

We drop

    # It's hard to find the actual address.
    # The rpc mgr addr attribute is a list.  Each element in the
    # list is a socket domain (AF_INET, AF_UNIX, etc.) and an
    # address.

note because at the time it was added (81f586c4) it came with

    addr = self._storage._rpc_mgr.addr[0][1]

but nowdays after 03867188 getting to server address is just by ClientStorage._addr.

/reviewed-on https://github.com/zopefoundation/ZEO/pull/170
/reviewed-by @d-maurer

kirr:

this fixes "uvloop"-flavour CI failures where things were failing to
install with

    RuntimeError: uvloop requires Python 3.7 or greater
    An error occurred when trying to install /tmp/tmph6b7orkdget_dist/uvloop-0.15.2.tar.gz. Look above this message for any errors that were output by easy_install.
    While:
      Installing test.
      Getting distribution for 'uvloop>=0.5.1'.

(see e.g. https://travis-ci.com/github/zopefoundation/ZEO/jobs/498522609)

I suspect the other underscore containing settings have the same typo - but I hadn't have time to check it, so I'm leaving it as a suggestion for further work. Sorry 'bout that.