Merge 4.x branch

Better support of the new API to notify of resolved conflicts (store/tpc_finish)

Conflicts:
	src/ZODB/interfaces.py

ZEO handles undo asynchronously and ZODB has for some time allowed
undo oids to be returned via vote.

Conflicts:
	src/ZODB/interfaces.py

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

When switching all storages to the new API in the master branch,
I found a few issues, mainly with blobs.

1. Created/modified blobs are invalidated during the first phase
   (in Connection._store_objects):

    obj._p_invalidate()

    And with the old API, _handle_serial then sets _p_serial
    Ghost objects are not supposed to have a _p_serial and with the new API,
    a few tests would fail because _p_serial are checked without activating the
    blob first.

2. Another consequence of _handle_serial not updating _p_changed/_p_serial
   immediately is that created objects are stored twice if __getstate__
   modifies itself. This case is tested in testConnection by
   doctest_lp485456_setattr_in_setstate_doesnt_cause_multiple_stores

    Hence the change in Connection._commit:
    - self._modified is already appended in Connection._store_objects
    - (obj._p_serial == z64) instead of (oid in self._creating)
      would not work for savepoints.

3. Setting _p_changed of a Blob with no uncommitted changes would cause an error
   (lp440234_Setting__p_changed_of_a_Blob_w_no_uncomitted_changes_is_noop)

    Fixed by the same change as in 2:
    - oid was appended twice to self._modified but reverted one
      (self._modified.pop() in _store_objects)
    - the test passed because _p_changed was reset early by _handle_serial

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.

Conflicts:
	CHANGES.rst
	setup.py

See #60, #65.

Modernize and cleanup locking

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.

- Move from try/finally to with

- Start phasing out the _lock_acquire/_lock_release shortcuts

- Replace simpler @locked decorators with with statements.

  I've come to prefer the with style tp the decorator style.
  I think it's clearer.

  (The decorators preceeded the Python with statement.)

  I left the decorators in cases where they were used with preconditions.

This is mostly pretty mechanical, although it got a little delicate in
places and ... tests

Move load to imvcc storage

Which led to me to call site that needed it to be removed.

And finally get rid of the version argument.

Also drop the version argument from IStorageWrapper.invalidate.