#5296 Add a flag for start_command, if true do not let start_command run

Add a flag to start_command to monitor when it can be run, if start_command is called while the flag is true, it will do nothing and let the user know that start is already running. The flag is set to true when start_command is run successfully then set to false when it is finished.

This is a hummingbot client UI improvement.

Link: https://github.com/hummingbot/hummingbot/pull/5296