Unbuilt one-hop circuits sometimes hang around forever

I've noticed that with tor 0.2.4.10-ish, Vidalia can accumulate a lot of "Path Empty" circuits after a while. According to my control port, these are ONEHOP_TUNNEL circuits that haven't yet connected to a guard. They seem to stick around for days unless I close them.

I suppose this could be due to the circuit build timeout changes in #7341 (moved), or Roger's "let circuits last forever if you have none open" hack from #3443 (moved), or directory guards, or $RANDOM_GUESS.

I'll poke at it with gdb next time I notice it.