The problem is that clients and hidden services are receiving
relay_early cells, and they tear down the circuit.

Hack #1 is for rendezvous points to rewrite relay_early cells to
relay cells. That way there are never any incoming relay_early cells.

Hack #2 is for clients and hidden services to never send a relay_early
cell on an established rendezvous circuit. That works around rendezvous
points that haven't upgraded yet.

Hack #3 is for clients and hidden services to not tear down the circuit
when they receive an inbound relay_early cell. We already refuse extend
cells at clients.