Implement graceful shutdown

[greeeen-dev]

One thing about Unifier is, it can't shut down using Ctrl+C if you have plugins like Guilded Support. Additionally, bridge cache won't be saved if periodic backups aren't enabled, which can be problematic.

To fix this, you should implement the shutdown logic in sysmgr, but replace the sys.exit(0) with the Pterodactyl quit command