tor-proto: Extract Create message handling from Reactor::run_once()
I was trying to grok how the circuit reactor works and I found myself moving some code around. The outcome is this MR, which I think makes the code in run_once()
somewhat easier to follow.
I think it's safe to handle ChanMsg::Create
separately, because
there's nothing for the reactor to do until the first hop of the circuit
is created (so blocking on this should be alright).