Allow worker tasks to have priorities

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