RFC: Provide and use SometimesUnboundedSender in circuit reactor

This is an attempt at #1419 (closed). It came out much nicer and smaller than I expected.

But: There aren't any unit tests for the new type. I wasn't quite sure how to structure them.

Also I'm not sure our integration tests are sufficient to catch circuit reactor bugs (eg, bugs I might introduce here) but I guess that's part of #1387 (closed) #1397 (closed)?

I believe my changes not to have any overall functional effect, but: I notice the comment in send_msg_direct that I'm deleting. If there were such a bug (someone calling send on the chan_sender directly) this MR would fix that bug in passing.

The branch is on top of !2171 (merged)