R1:a5fb482a006b

Add a ref cache and index construction to Diviner

Summary: Cache refs in a single large index; rebuild the main index from them.

Test Plan: {F32334}

Reviewers: btrahan, vrana, chad

Reviewed By: chad

CC: aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D4900