Specify client-side behavior around re-fetching hsdescs and looking at revision counters
See discussion at arti#913 (closed) point 7.
There are two questions that we should specify here:
- When should a client discard an onion service descriptor that it already has, and/or look for a new one?
- What (if anything) should a client check about the new descriptor's revision counter?
The first question has a fair amount of discussion at the above-linked ticket, but the second one not so much. Should we be watching out for attempts to roll us back to an superseded descriptor?