Overhaul Tracker Shutdown

[josecelano]

Relates to: https://github.com/torrust/torrust-tracker/issues/1477 Depends on: https://github.com/torrust/torrust-tracker/issues/1405

See point 3 (Future Improvements) in the issue https://github.com/torrust/torrust-tracker/issues/1477.

The main idea is to capture that Ctrl+c signal only in the main function and let the main app handle it. It should be propagated on cascade to all jobs.

It depends on other issues, such as https://github.com/torrust/torrust-tracker/issues/1405, and may require similar changes to other spawned tasks.

cc @da2ce7