ASGI: Add affordances and recipe for long-polling

The 3.0 ASGI implementation already affords long-polling, but it would be useful to provide a recipe to demonstrate the pattern in the context of a Falcon app.

Also, for the sake of efficiency, we may want to provide a req.wait_disconnect() or similar method that can be executed in a background task within a responder. The responder could then use this in order to detect if it should abandon a long-poll early in the case that the client bails out. This is similar to what asgi.App does when streaming SSE's (using watch_disconnect().