Registers a handler to receive flow processing messages.

def _RegisterFlowProcessingHandler(self, handler):
    """Registers a handler to receive flow processing messages."""
    self.flow_handler_stop = False
    self.flow_handler_thread = threading.Thread(
        name="flow_processing_handler",
        target=self._HandleFlowProcessingRequestLoop,
        args=(handler,))
    self.flow_handler_thread.daemon = True
    self.flow_handler_thread.start()