Optimize reindex_everything.php

Summary:
We have two troubles with this script:

1. Our revisions and commits don't fit in the memory. (Our tasks do :-).)
2. Reindexing revisions is slow.

Test Plan: Ran it.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3483