Future of on_startup/on_shutdown and lifespan

[adriangb]

Background:

• Further develop startup and shutdown events tiangolo/fastapi#617
• Add support for contextlib-like lifecycle handlers. #785
• Support generator function to manage startup/shutdown. #799

I don't think we've been explicit yet as to what the planned future state is for lifespan and on_startup / on_shutdown. We currently have two mutually exclusive ways to do things, which is confusing for users and adds complexity to our codebase.

Using async context managers is (as far as I can tell) never worse than on_startup / on_shutdown and makes some use cases much easier (if you need to wrap another context manager like httpx.AsyncClient). Also as discussed in tiangolo/fastapi#617 using on_startup / on_shutdown has some serious foot guns.

Given that using async context managers are all around better, I propose we get rid of on_startup and on_shutdown for 1.0.

What do other think?

cc @graingert @tomchristie @Kludex