`ClientCache._oid_dict` shall not have empty values. For a given oid, when the
last item is removed from the cache, the oid must be removed as well to free
memory. In some cases, this was not done.

A consequence of this bug is the following exception:

    ERROR ZODB.Connection Couldn't load state for 0x02d1e1e4
    Traceback (most recent call last):
      File "ZODB/Connection.py", line 860, in setstate
        self._setstate(obj)
      File "ZODB/Connection.py", line 901, in _setstate
        p, serial = self._storage.load(obj._p_oid, '')
      File "neo/client/Storage.py", line 82, in load
        return self.app.load(oid)[:2]
      File "neo/client/app.py", line 358, in load
        self._cache.store(oid, data, tid, next_tid)
      File "neo/client/cache.py", line 228, in store
        prev = item_list[-1]
    IndexError: list index out of range