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