Allow serial to be returned as late as tpc_finish

This makes possible for storage to allocate serial inside tpc_finish,
removing the requirement to serialise 2PC's second phase phase (tpc_vote
to tpc_finish/tpc_abort).
Co-Authored-By: Julien Muchembled <jm@nexedi.com>

This also reverts commit 4905bb85:
sleeping 1ms is enough.

Fix winbot failures

Thanks

This fixes the following random failure:

Failure in test checkBaseHistory (ZODB.tests.testDemoStorage.DemoStorageTests)
Traceback (most recent call last):
  File "unittest\case.py", line 384, in _executeTestPart
    function()
  File "ZODB\tests\testDemoStorage.py", line 96, in checkBaseHistory
    self._checkHistory(base_only())
  File "ZODB\tests\HistoryStorage.py", line 46, in _checkHistory
    self.assertLess(a, b)
  File "unittest\case.py", line 1026, in assertLess
    self.fail(self._formatMessage(msg, standardMsg))
  File "unittest\case.py", line 508, in fail
    raise self.failureException(msg)
AssertionError: 1465433823.260304 not less than 1465433823.260304

Conflicts:
	CHANGES.rst

Cuz I neglected to push when I made 4.3.0. Thanks DCVS!

Fixed: FileStorage loadBefore didn't handle deleted/undone data correctly

Enforce minimum required Python versions: 2.7, 3.3.

+1

Merge pull request #62 from zopefoundation/dont-depend-on-implementation-detail-in-tpc_finish-test-assertions

Fixed a test that depended on some off behavior of tpc_finish in some…

Closes #59.

This answers #13

- It failed with oids that already exist in the base storage.
- MappingStorage returned timestamps in wrong format.

On Python 2 and Mac OS X, checkFlushAfterTruncate was failing.

Changed to always empty the file pool.  I doubt that just flushing on
Python 2 was much of an optimization, and ... Python 2. :)

Incididentally, should make the Py26 winbot happy too, until we can get
it disabled.

Fixed test failure introduced in pull-request 51

Fix possible data corruption after FileStorage is truncated to roll back a transaction

The PR depended on system time being UTC.

Multi-threaded IO support, which is new to ZODB 3.10, allows clients to read
data (load & loadBefore) even after tpc_vote has started to write a new
transaction to disk. This is done by using different 'file' objects.

Issues start when a transaction is rolled back after data has been appended
(using the writing file object). Truncating is not enough because the FilePool
may have been used concurrently to read the end of the last transaction:
file objects have their own read buffers which, in this case, may also contain
the beginning of the aborted transaction.

So a solution is to invalidate read buffers whenever they may contain wrong
data. This patch does it on truncation, which happens rarely enough to not
affect performance.

We discovered this bug in the following conditions:
- ZODB splitted in several FileStorage
- many conflicts in the first committed DB, but always resolved
- unresolved conflict in another DB
If the transaction is replayed with success (no more conflict in the other DB),
a subsequent load of the object that could be resolved in the first DB may, for
example, return a wrong serial (tid of the aborted transaction) if the layout
of the committed transaction matches that of the aborted one.

The bug usually manifests with POSKeyError & CorruptedDataError exceptions in
ZEO logs, for example while trying to resolve a conflict (and restarting the
transaction does not help, causing Site Errors in Zope). But theorically,
this could also cause silent corruption or unpickling errors at client side.

Cleanup dependencies, remove ZEO leftovers