Provide and implement SinkCloseChannel

The branch is on top of !2488 (merged) !2468 (merged) which is approved but has not yet merged at the time of writing

This is needed for the same reasons as !2468 (merged) !2488 (merged)'s SinkTrySend. Happily it's a lot simpler.

At the end of the branch, change the two call sites in tor-proto.

Part of #351 (closed)

I'll assign review to @ahf manually, since he did that other MR.