Move Phage to FuturePool

Summary: Ref T11968. Phage has another "sustained pool of Futures" use case, and needs some slight adjustments after Future API changes.

Test Plan: Ran bin/phage status ..., got a clean result instead of a JSON decoding failure.

Maniphest Tasks: T11968

Differential Revision: https://secure.phabricator.com/D21058