Support generator function to manage startup/shutdown.

[tomchristie]

An alternate implementation of #785

• Refactor lifespan to run as an async generator
• Allow lifespan_context=... argument to Starlette and Router.
• Add app argument to the lifespan context function.

End result API would be allowing the application to take a single async generator function, that can encapsulate any required startup/shutdown logic.

Taking the example in #785, you'd end up with this kind of pattern...

async def lifespan(app):
    async with db.ConnectionPool() as app.state.pool:
        yield

app = Starlette(routes=..., middleware=..., lifespan=lifespan)

[sm-Fifteen]

Using this approach, I have two questions:

1. Couldn't an application use a sync generator instead of an async one as their lifespan context? After all, servers are forbidden from accepting connections until the a startup complete event is recieved, so there isn't a whole lot of things that could preempt the startup coroutines regardless of whether the functions being called are blocknig or not. There might not be any async code to run on startup or shutdown.
2. Could this be a step in fixing lifespan events in sub-applications #649?

[tomchristie]

1. The async function is sufficient, since it can run either sync or async code. It might be graceful to be able to have it accept either kind of function, or it might be unnecessary extra noise?
2. Potentially, yeah.

[sm-Fifteen]

1. The async function is sufficient, since it can run either sync or async code. It _might_ be graceful to be able to have it accept either kind of function, or it might be unnecessary extra noise?

Having looked into it, you can't loop though a sync and an async iterator with the same kind of loop, so it would mean duplicating the iteration part, which would be unneccessarily noisy. It probably wouldn't be worth it anyway, so nervermind that.

2. Potentially, yeah.

That's good if it turns out that way, lifespan has a number of caveats that make it unreliable (#649) or impractical (what this PR attempts to fix) to use right now. That's probably out of scope for this PR, though.

[iksteen]

Works for me, just be sure to properly document that lifespan and startup/shutdown are mutually exclusive (at least, in this implementation).

[tomchristie]

Couldn't an application use a sync generator instead of an async one as their lifespan context?

Alrighty, for completeness I've added support for the function to be either a generator or an async generator.

[rugleb]

When can we expect a release with this feature?