Add PID file to prevent double execution

[duffyjp]

There is currently nothing stopping you from running the script multiple times. Adding a PID check can prevent that, and also allow you to cron a startup every hour or something which would just exit if it's already running.

[duffyjp]

🌶  rb2db 14:43$ exe/rb2db 
[!] rb2db already running on pid 27781