Refactor tor_proto::channel::Reactor to use an UnboundedSender

etarequested to merge
eta/arti:218-1 into main

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))

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