Hello!

I was looking at the Gravatar code and noticed Larsi's change last year
to disable the URL caching and instead use a hash table. I would like to
see the timeout for the hash table configurable again and this sent me
to the `url-cache-expired' and `url-cache-prune-cache' functions. It
appears that `url-cache-expired' is no longer used after the Gravatar
caching change, so I was wondering if maybe it could be useful to have a
more general, URL-specific expiry policy in the URL caching code. This
way, on-disk caching for, e.g., gravatar.com could be longer than for
the in-memory hash - right now expiry for the Gravatar memory cache is
12 hours, for the global disk cache it is 1 hour.

I wrote up the below and I am asking for feedback on it. I was
particularly concerned about back-forming the URL based on the cache
path and there really should be some test cases around that. I have done
minimal testing and I am open to suggestions to do this differently.