Add server hooks

[abersheeran]

Similar to Gunicorn.

This will solve more problems for us. For example https://stackoverflow.com/questions/58133694/graceful-shutdown-of-uvicorn-starlette-app-with-websockets . If we provide a before_shutdown hook, we can handle it gracefully without dangerously modifying uvicorn's internal code.

This is just an idea at the moment.

@encode/maintainers Do anybody have any thoughts?

[Kludex]

If we add server hooks, like post_request, we can use it for custom access log messages.

[abersheeran]

Yeah.

[Kludex]

@tomchristie What do you think?

[tomchristie]

I'm unclear what functionality this supports that isn't already covered by the ASGI messaging.

(I might not be the best person to ask tho)