code: def set_querier_mode(self, dpid, server_port):
          
          self.dpid = dpid
          self.server_port = server_port
          if self._querier_thread:
              hub.kill(self._querier_thread)
              self._querier_thread = None
docs: set the datapath to work as a querier. note that you can set
          up only the one querier. when you called this method several
          times, only the last one becomes effective.