When ConnectionPool._initNodeConnection fails a first time with:

  StorageError: protocol error: already connected

the following assertion failure happens when trying to reconnect before the
previous connection is actually closed (currently, only the node sending an
error message closes the connection, as commented in EventHandler):

  Traceback (most recent call last):
    File "neo/client/Storage.py", line 82, in load
      return self.app.load(oid)[:2]
    File "neo/client/app.py", line 367, in load
      data, tid, next_tid, _ = self._loadFromStorage(oid, tid, before_tid)
    File "neo/client/app.py", line 399, in _loadFromStorage
      askStorage)
    File "neo/client/app.py", line 293, in _askStorageForRead
      conn = cp.getConnForNode(node)
    File "neo/client/pool.py", line 98, in getConnForNode
      conn = self._initNodeConnection(node)
    File "neo/client/pool.py", line 48, in _initNodeConnection
      dispatcher=app.dispatcher)
    File "neo/lib/connection.py", line 704, in __init__
      super(MTClientConnection, self).__init__(*args, **kwargs)
    File "neo/lib/connection.py", line 602, in __init__
      node.setConnection(self)
    File "neo/lib/node.py", line 122, in setConnection
      attributeTracker.whoSet(self, '_connection'))
  AssertionError