Implement a directory-based cache

Summary: Implements a directory-based disk cache. This cache is more suitable for slow/large operations than the existing OnDisk cache. Specifically, it is suitable for use in Diviner.

Test Plan: Unit tests.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D4288