Reverse meaning of task priority column

Summary:
Ref T6615. Mixing ASC and DESC ordering on a multipart key makes it dramatically less effective (or perhaps totally ineffective).

Reverse the meaning of the `priority` column so it goes in the same direction as the `id` column (both ascending, lower values execute sooner).

Test Plan:
  - Queued 1.2M tasks with `bin/worker flood`.
  - Processed ~1 task/second with `bin/phd debug taskmaster` before patch.
  - Applied patch, took ~5 seconds for ~1.2M rows.
  - Processed ~100-200 tasks/second with `bin/phd debug taskmaster` after patch.
  - "Next in Queue" query on daemon page dropped from 1.5s to <1ms.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: aklapper, 20after4, epriestley

Maniphest Tasks: T6615

Differential Revision: https://secure.phabricator.com/D10895