Garbage collect TTL'd cache entries from the general cache

Summary: We currently garbage collect general cache entries after a set period of time (30 days by default), but the recent changes to DarkConsole have left us writing a lot of large, short-TTL data to the cache. In addition to a maximum age, GC cache entires after they TTL out.

Test Plan: Ran GC daemon, saw TTL'd entries get collected. Inserted a TTL'd entry, saw it get collected by GC. Saw non-ttl'd entries not get collected.

Reviewers: chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D4990