Merge proposals 176 and 184(v3 handshake) into tor-spec.txt
We've implemented proposal 176 (for the v3 handshake) and 184 (for the v3 link protocol). We should merge them into tor-spec.txt.
I've written up a candidate merge as branch "merge_v3" in my public torspec repo, but it is complicated enough that I would like some review.