Add on_stopping hook to set anything before master is killed

[ilpianista]

As suggested in #2944. My use case is to postpone the shutdown of gunicorn.

[ilpianista]

@benoitc may you have a look at this please? Thanks

[pajod]

Design consideration: Whether the

• failure of this hook should be full responsibility of the hook author (more flexible, wrapping try-except if so desired), or whether
• failure of this hook should be ignored, shutdown proceeding anyway (similar to post_request() hook, within which exceptions are not treated as fatal)