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