Duplicate rendezvous cookie in ESTABLISH_RENDEZVOUS

Seems like people have started receiving this Duplicate rendezvous cookie in ESTABLISH_RENDEZVOUS warning message on their relays: https://lists.torproject.org/pipermail/tor-relays/2016-October/010535.html

This might be a logic bug in tor, or it might be that Tor clients (or an alt implementation) is reusing the same rend point and same rend cookie multiple times, which simply does not work.

(Meta: Unclear what Points should be in this ticket)