Registers a handler to receive flow processing messages.

def RegisterFlowProcessingHandler(self, handler):
    """Registers a handler to receive flow processing messages."""
    self.UnregisterFlowProcessingHandler()

    if handler:
      self.flow_processing_request_handler_stop = False
      self.flow_processing_request_handler_thread = threading.Thread(
          name="flow_processing_request_handler",
          target=self._FlowProcessingRequestHandlerLoop,
          args=(handler,))
      self.flow_processing_request_handler_thread.daemon = True
      self.flow_processing_request_handler_thread.start()