Refactor tor_proto::channel::Reactor to use an UnboundedSender
There wasn't any good reason for tor-proto's channel reactor to use a
shedload of oneshot channels instead of just an mpsc UnboundedSender,
and the whole CtrlResult
thing made even less sense.
Straighten this code out by replacing all of that machinery with a simple UnboundedSender, instead.
(part of #218 (closed))