R1:b5f7e9eec60a

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.