Create server util to allow pulling requests

[seanmonstar]

The current server API is callback-based, where Service is called whenever a request is received. Some applications need to invert it such that they take a request only when they are ready. This proposes creating a utility in hyper-util that would allow usage similar to this:

// naming is hard
let (svc, rx) = hyper_util::server::puller();

let conn = Http::new().serve_connection(socket, svc);
tokio::spawn(conn);

// pull off requests
while let Some((req, tx)) = rx.next().await? {
    println!("{:?}", req);
    tx.respond(Response::new(Empty::new());
}

[seanmonstar]

@piscisaureus I think this would make it easier to fulfill the pattern you want that we discussed, yea?

[piscisaureus]

Excellent! This would go a long way.

Another integration pain point is to gracefully close the connection (stop taking new requests while finishing already existing ones). This also requires a complicated kludge - see https://github.com/denoland/deno/blob/8eed24cd3d9cea5179c65e43d23fbf6cf4d873ca/ext/http/lib.rs#L146.
(I could understand if you considered this out of scope for this issue).