Endpoints return a deferred.
Remove debugging code.
Refactor; fix some major bugs.

* RTT was being recorded for non-ACK messages, causing inflated average RTTs.
* Sent messages and queued messages were all hells of conflated.
Add resending; a minimal state machine.
Fix up resolving curvecp streams.
Remove some debugging code.
Add reads closing state.
Cancel the cookie MultiTimeout.
Simplify some logic.
Delay the end of the stream for acks.
Make things more testable.