Rendezvous points are too strict about RENDEZVOUS1 payload length

In rend_mid_rendezvous() we do

  if (request_len != REND_COOKIE_LEN+DH_KEY_LEN+DIGEST_LEN) {
    log_fn(LOG_PROTOCOL_WARN, LD_PROTOCOL,
         "Rejecting RENDEZVOUS1 cell with bad length (%d) on circuit %u.",
         (int)request_len, (unsigned)circ->p_circ_id);
    reason = END_CIRC_REASON_TORPROTOCOL;
    goto err;
  }

which is sad, because it locks us in to a particular RENDEZVOUS2 payload format. In the future, we want to be able to use other payload formats. So we should stop rejecting RENDEZVOUS1 cells with other payload lengths.