Queue search indexing tasks at a new PRIORITY_INDEX, not PRIORITY_IMPORT

Summary:
Depends on D20175. Ref T12425. Ref T13253. Currently, importing commits can stall search index rebuilds, since index rebuilds use an older priority from before T11677 and weren't really updated for D16585.

In general, we'd like to complete all indexing tasks before continuing repository imports. A possible exception is if you rebuild an entire index with bin/search index --rebuild-the-world, but we could queue those at a separate lower priority if issues arise.

Test Plan: Ran some search indexing through the queue.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13253, T12425

Differential Revision: https://secure.phabricator.com/D20177