tor-proto::circuit::StreamMap: Use StreamPollSet

• Refactors StreamMap to use StreamPollSet to manage its receivers for mpsc streams.
• Extends StreamMap to support iterating only over streams that have a pending outgoing message, and in round-robin order.
• Updates circuit::reactor::Reactor to use this functionality. It now iterates only over streams that have a ready outgoing message, and only actually "pops" a message that is ready to be sent.

This mildly simplifies the circuit reactor, but more importantly clears the way to:

• Remove the "outbound queue" of messages that were pulled from stream channels but that we couldn't send yet due to congestion control.
• Support opportunistic packing when preparing to send a relay message. (proposal 340).
• Refactor the circuit reactor's run_once into futures that we can select! over.

Progress on #1421 (closed), #1397 (closed), #1067