hs: Don't bruteforce every pending connection when we get a RENDEZVOUS ack

In the v2 subsystem, we have this:

  /* XXXX++ This is a pretty brute-force approach. It'd be better to
   * attach only the connections that are waiting on this circuit, rather
   * than trying to attach them all. See comments bug 743. */
  /* If we already have the introduction circuit built, make sure we send
   * the INTRODUCE cell _now_ */
   connection_ap_attach_pending(1);

And v3 is also doing that (legacy/trac#17242 (moved)) so we should find another approach to this instead of scanning the entire set of pending connections.