Add PhutilKeyValueCache and some implementations

Summary:
This is basically a slightly-more-abstract version of Facebook's cache stack, without any of the cachekey callbacks (in SocialBB, I implemented something similar above this).

• PhutilKeyValueCache defines an abstract key-value cache interface with get/set/delete operations.
• PhutilKeyValueCacheStack allows you to compose several caches so they behave like a single cache with readthrough semantics (like request -> APC -> Memcache at Facebook).

Then there are cache implementations:

• PhutilKeyValueCacheAPC implements an APC-based cache.
• PhutilKeyValueCacheMemcache implements a memcache-backed cache.
• PhutilKeyValueCacheInRequest is like Facebook's $GLOBALS['CACHE'] (i.e., kind of a bad idea).
• PhutilKeyValueCacheOnDisk implements a disk-based cache. With clever use of getmypid() / posix_getppid() and such we can reasonably fake something like APC for non-APC installs.

Test Plan: Ran unit tests. This is a slightly modified version of code that worked correctly in SocialBB.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2062

Differential Revision: https://secure.phabricator.com/D3974