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 (#17242 (moved)) so we should find another approach to this instead of scanning the entire set of pending connections.