Improve Differential query plans

Summary:
Ref T8575. We run a big "(A) UNION (B)" query on the home page and on the main Differential page.

"A" can always be improved by using %Ls, so it can use the second half of the (authorPHID, status) key.

"B" can sometimes be improved if the fraction of open revisions is smaller than the fraction of revisions you are reviewing. This is true for me on secure.phabricator.com (I'm a reviewer, either directly or via 'Blessed Reviewers', on about 80% of revisions, but <5% are open). In these cases, a (status, phid) key is more efficient.

Test Plan: Tweaked queries and added keys on this server, saw less derpy query plans and performance.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T8575

Differential Revision: https://secure.phabricator.com/D13325