Move task leasing to a dedicated query

Summary: This simplifies the fairly thorny logic of leasing tasks a bit. I'm planning to introduce another callsite shortly for Drydock.

Test Plan: Ran bin/phd debug taskmaster, observed sensible queries and correct operation.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2015

Differential Revision: https://secure.phabricator.com/D3855