Feature request: on_shutdown for closing gracefully

[mmiller-max]

Add similar functionality to aiohttp's on_shutdown function.

This would enable users to specify custom shutdown functions (for example close database connections) that would trigger when an HTTP server stops. I'm not sure on the how its routed through, but it would be great if this could be triggered when, for example, CMD+Cing a Docker container which is running a Julia App hosting a HTTP server.

I'm happy to work on this and will give it a look later, but if anyone has any pointers that would be great.