Improve performance of differential.query

Summary:

• Instead of running (1 + N * 4) queries, run 5 queries.
• Add 'dateModified' field.

Test Plan:

• Ran various queries against various revisions, data seemed unchanged.
• Checked query log.

Reviewers: nh, jonathanhester, jungejason, btrahan

Reviewed By: jonathanhester

CC: aran, epriestley, jonathanhester, jungejason

Differential Revision: 1252