Implement counter galois onion, relay side in C Tor

See arti#1936 (closed).

For Arti relays to get utility out of Counter Galois Onion, there will need to be relay support. For now, Arti relays are still a work in progress, so we'll likely need support in C tor.

This will be harder than the arti implementation; I am especially concerned about the new relay cell format.