Feature request #1695: Caching notes for repo browser

Probably going to have one of three things:

• a post-receive hook that puts a file with a timestamp in it in each repo
• a post-receive hook which does cache invalidation in script/runner style
• a post-receive hook that writes the timestamp to the Repo model instance in the database itself (again, script/runner style, or perhaps via an internal-only HTTP API)

If we use the timestamp approach, memcached is our friend here, because we'll be able to just store everything in memcached with a timestamp, and when we update the repo the timestamp will change and the older records will eventually get pushed out of the cache with no explicit invalidation required.

Note that at the moment everything can be cached except or the repos#show view, which is the only page which shows admin-specific content (the R/W clone URL).

See the original repo-browing ticket, ticket #1135, for additional notes.