Implement a more compact, general database-backed key-value cache

Summary:
See discussion in D4204. Facebook currently has a 314MB remarkup cache with a 55MB index, which is slow to access. Under the theory that this is an index size/quality problem (the current index is on a potentially-384-byte field, with many keys sharing prefixes), provide a more general index with fancy new features:

• It implements PhutilKeyValueCache, so it can be a component in cache stacks and supports TTL.
• It has a 12-byte hash-based key.
• It automatically compresses large blocks of data (most of what we store is highly-compressible HTML).

Test Plan:

• Basics:
  • Loaded /paste/, saw caches generate and save.
  • Reloaded /paste/, saw the page hit cache.
• GC:
  • Ran GC daemon, saw nothing.
  • Set maximum lifetime to 1 second, ran GC daemon, saw it collect the entire cache.
• Deflate:
  • Selected row formats from the database, saw a mixture of 'raw' and 'deflate' storage.
  • Used profiler to verify that 'deflate' is fast (12 calls @ 220us on my paste list).
• Ran unit tests

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D4259