Define a new interface for storages that support the new commit protocol.

This allows interface incompatibilities:

- Future version of ZODB that requires the new protocol being
  configured with a storage that doesn't implement the new protocol

- ZEO has a client that uses the old protocol can't use a storage that
  implements the new protocol.

To be caught early.

This commit also clarifies that the serial returned by ``undo()`` is ignored.

This reverts commit 7901868a.

This allows interface incompatibilities:

- Future version of ZODB that requires the new protocol being
  configured with a storage that doesn't implement the new protocol

- ZEO has a client that uses the old protocol can't use a storage that
  implements the new protocol.

To be caught early.

This commit also clarifies that the serial returned by ``undo()`` is ignored.

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