text: Title: ASGI: Add affordances and recipe for long-polling
  Body: 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()`.