Stop using FuturesUnordered<onshot::Receiver<...>> in tor-guardmgr

In !118 (merged), @eta simplified some code in tor-proto that used a FuturesUnordered stream of oneshot::Receiver. Thanks to her patch, now it just uses an UnboundedReceiver.

We should do the same in tor-guardmgr. I'm assigning this to myself so I get some practice with the idiom too.