Merged requested to merge mirrorauth-cache-tweaks into master
The first commit adjusts the cache key to not include the original request URI.  The second one tweaks how cache entries are populated and refreshed to avoid hitting archweb multiple times while entries are inserted or updated (if stale). 
I have tested and deployed these changes. Seems to work well!
 this gets us from many auth requests (1 per URL) down to a handful (however many pacman does concurrently)
 after this I now only notice at most one auth request per 5 minutes for both nonexistent and stale cache entries