R1:9a679bf3746c

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…