This fixes invalid next_serial entries in cache,
and the following error for values not in cache:

  Traceback (most recent call last):
    File "ZODB/Connection.py", line 856, in setstate
      self._setstate(obj)
    File "ZODB/Connection.py", line 894, in _setstate
      self._load_before_or_conflict(obj)
    File "ZODB/Connection.py", line 922, in _load_before_or_conflict
      if not self._setstate_noncurrent(obj):
    File "ZODB/Connection.py", line 945, in _setstate_noncurrent
      assert end is not None
  AssertionError