Graceful shutdown nginx and uwsgi

[desaintmartin]

Currently, when the container is stopped, supervisor sends a SIGTERM signal to both nginx and uwsgi.

By default, on nginx, it causes a fast shutdown (not honoring open requests), and on uwsgi it causes a brutal kill of all workers.

This PR makes supervisor send a SIGQUIT to nginx (graceful stop), and add a built-in handler in uwsgi to gracefully stop workers.

Results: no more errors when doing a kubernetes rolling-release for servers with frequent long-requests.

Caveats: you need "master" mode in uwsgi to be enabled (true by default).

[tiangolo]

Great job @desaintmartin !

And great research, the hook-master-start = unix_signal:15 gracefully_kill_them_all was hard to find, quite "undocumented" in uWSGI.

Thanks a lot! 🎉