Allow taskmaster daemons to hibernate

Summary: Ref T12298. Like PullLocal daemons, this allows the last daemon in the pool to hibernate if there's no work to be done, and awakens the pool when work arrives.

Test Plan:

• Ran bin/phd debug task --trace.
• Saw the pool hibernate and look for tasks.
• Commented on an object.
• Saw the pool wake up and process the queue.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12298

Differential Revision: https://secure.phabricator.com/D17559