Cache items are stored in double-linked chains. In order to quickly know the
number of history items, an extra attribute is used to count them. It was not
always decremented when a history item was removed.

This led to the following exception:
  <ClientCache history_size=100000 oid_count=1959 size=20970973 time=2849049 queue_length=[1, 7, 738, 355, 480, 66, 255, 44, 3, 5, 2, 1, 3, 4, 2, 2] (life_time=10000 max_history_size=100000 max_size=20971520)>
  poll raised, retrying
  Traceback (most recent call last):
    ...
    File "neo/client/handlers/master.py", line 137, in packetReceived
      cache.store(oid, data, tid, None)
    File "neo/client/cache.py", line 247, in store
      self._add(head)
    File "neo/client/cache.py", line 129, in _add
      self._remove(head)
    File "neo/client/cache.py", line 136, in _remove
      level = item.level
  AttributeError: 'NoneType' object has no attribute 'level'