memcache: Make minimum size for compression configurable per chain.

This allows us to trade off CPU time for bandwidth / cache space in
different amounts on different cache chains since they have distinct
behaviours.
memcache: Reduce compression threshold for page/rendercache.
memcache: Reduce compression threshold for permacache.
memcache: Reduce compression threshold for main caches.
Add a tool for generating subreddit traffic reports.
Add author_slow property to Message.