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 arti#218)
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