I am learning about the Gazebo elevator plugin with the worlds/elevator.world example. I wrote a program to convert ROS String messages into gz_string and publish the converted messages to topic ~/elevator, then I was able to move the elevator around by running rostopic pub command. However, I noticed that I could only send a new elevator request after the elevator door had been filled closed. Is there some way I can send multiple elevator requests within a short interval and let the plugin queue the messages to handle them later?