R1:aae5f9efd3d3

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…