Relays should accept RENDEZVOUS1 cells of variable size

In rend_mid_rendezvous, we require that the RENDEZVOUS1 cell has precisely DH_KEY_LEN+DIGEST_LEN bytes of handshake info, which is passed verbatim to the service in the RENDEZVOUS2 cell.

For prop 224, we want to use these same cells. Prop 224 redefines RENDEZVOUS1 to be:

RENDEZVOUS_COOKIE [20 bytes]
HANDSHAKE_INFO [variable; to end of cell]

Prop 224's HANDSHAKE_INFO is much shorter. To remain compatible with older RPs, we will need to pad these cells in prop 224. Still, we should start accepting RENDEZVOUS1 cells of any size, and forward everything after the cookie verbatim in the RENDEZVOUS2 cell.