Summary: Fixes T5336. Currently, `PhabricatorWorkerLeaseQuery` is basically FIFO. It makes more sense for the queue to be a priority-queue, and to assign higher priorities to alerts (email and SMS).
Test Plan: Created dummy tasks in the queue (with different priorities). Verified that the priority field was set correctly in the DB and that the priority was shown on the `/daemon/` page. Started a `PhabricatorTaskmasterDaemon` and verified that the higher priority tasks were executed before lower priority tasks.
Reviewers: epriestley, #blessed_reviewers
Reviewed By: epriestley, #blessed_reviewers
Subscribers: epriestley, Korvin
Maniphest Tasks: T5336
Differential Revision: https://secure.phabricator.com/D9871
