Rendezvous failures could distinguish "unknown cookie" from "outdated cookie"
dgoulet spoke of getting a bunch of these on his relay: "(protocol warn): [warn] Rejecting RENDEZVOUS1 cell with unrecognized rendezvous cookie"
His theory was that maybe jerks are sending introduction requests when they never made the rendezvous circuit at all (and #25066 (moved) would address this possibility), but I think the explanation is much simpler: a real client had the rendezvous point established, but closed it (gave up) before the service got around to trying to connect to it.
This problem happens because of a timing issue, and when the network or the onion service is under load, the timing issue gets worse, because it takes longer between the introduction attempt and the rendezvous response.
So the patch to consider here is: if we remembered recently-established cookies, we could distinguish between "unknown cookie" or "cookie that would have worked earlier but it's too late now". That would help us know what the problem actually is right now, and also if these messages ramp up suddenly, it would let us distinguish which situation is causing it.