Fix daemon task queue to respect task priority

Summary:
Fixes an issue with T5336 / D9871. We did 99% of the work here but didn't actually turn on the priority sorting. The unit test passed by default, which didn't catch this.

  - Fix the unit test (it failed).
  - Fix the query (test now passes).
  - Add a "Next in Queue" element to the UI to make this kind of thing easier to spot/understand.

Test Plan: Ran unit test. Viewed "Next in Queue". Queued some tasks, flushed the queue. Web UI tracked the state sensibly.

Reviewers: joshuaspence, btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Differential Revision: https://secure.phabricator.com/D10766