This fixes a reqression in commit c39d5c67,
which could leads to failures like:

2016-11-29 09:56:58,756 ERROR ZODB.Connection Couldn't load state for 0x4843
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 352, in load
    data, tid, next_tid, _ = self._loadFromStorage(oid, tid, before_tid)
  File "neo/client/app.py", line 372, in _loadFromStorage
    for node, conn in self.cp.iterateForObject(oid, readable=True):
  File "neo/client/pool.py", line 91, in iterateForObject
    pt = self.app.pt
  File "neo/client/app.py", line 146, in __getattr__
    return self.__getattribute__(attr)
AttributeError: 'Application' object has no attribute 'pt'