Better solution for an HS client descriptor cache entry to expire

With #16389 (moved), we've added a 5 minutes timeout for an entry in the client descriptor cache to expire which is not ideal (details here https://trac.torproject.org/projects/tor/ticket/16389#comment:16).

One solution would be to add a counter in the hidden service descriptor that changes at with a new descriptor. To quote arma's in comment above:

Maybe this argues for hidden services putting the nonce into their HS descs
anyway, and publishing an updated HS desc every time they lose contact with
their intro points, to give clients a way to recognize when the HS has
acknowledged that things changed? Yuck.