code: def queue_callback(self, session, block_id, data):
          
          self._queue.put((session, block_id, data))
docs: Queues up a callback event to occur for a session with the given
          payload data.  Will block if the queue is full.
  
          :param session: the session with a defined callback function to call.
          :param block_id: the block_id of the message received.
          :param data: the data payload of the message received.