Optimize loading draft revisions

Summary:
This is killing us since D3615.

Maybe we should also change differential_inlinecomment index <commentID> to <commentID, authorPHID>.

Test Plan: Checked queries at /differential/ with comment drafts.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3667