tor-proto: simplify `ConfluxSet::circuit_action`

This simplifies the call signature of ConfluxSet::circuit_action, and renames it to ConfluxSet::next_circ_action. It now returns a Future instead of a Stream since we only ever use the first item in Reactor::run_once.

@gabi-250 I think these changes make things a bit nicer, but these changes are all opinionated, and I don't feel strongly either way, so feel free to veto them or suggest changes if you don't think they improve things.

/cc @dgoulet