This crate provides a generic implementation for request/response
protocols, whereby each request is sent on a new substream.

  1. Implement a custom ProtocolsHandler instead of using
     the OneShotHandler for better control and error handling.
     In particular, all request/response sending/receiving is
     kept in the substreams upgrades and thus the background
     task of a connection.
  2. Support multiple protocols (usually protocol versions)
     with a single `RequestResponse` instance, with
     configurable inbound/outbound support.

Co-authored-by: Toralf Wittner <tw@dtex.org>

Apparently the compiler just needs some help with the scope
of borrows, which is unfortunate.

Also expand the commentary on `send_response` to indicate that
responses may be discard if they come in too late.

As an analogue of `ResponseChannel::is_open` for outbound requests.

…sponse

Since `libp2p-request-response` is no longer using it.