Clean up Key-Value Cache diff

Summary:
Feedback from D3974.

• Make $profiler private and provide getProfiler().
• Document PhutilKeyValueCache.
• Document $ttl.
• Move $ttl from getKey() signature to setNextTTL($ttl) on PhutilKeyValueCacheStack.
• Make return values for kvimpl methods more consistent.
• Fix apc_clear_cache() -> apc_clear_cache('user').
• Test for invalid memcache TTL.
• Document memcache server specification.
• Use PhutilFileLock to control access to the cachefile in PhutilKeyValueCacheOnDisk.
• Write the new disk cache to the same place as the old disk cache. The write is now protected by a lock.
• Document PhutilKeyValueCacheStack->setCaches().
• Remove redundant method definitions in PhutilKeyValueCacheStack.
• Unrelated, update documentation of PhutilFileLock.

Test Plan: Ran unit tests. Ran slow TTL unit tests.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T2062

Differential Revision: https://secure.phabricator.com/D3978