Allow the PullLocal daemon to hibernate, and wake it when repositories need an update

Summary: Ref T12298. This allows the PullLocal daemon to hibernate like the Trigger daemon, but automatically wakes it back up when it needs to do something.

Test Plan:

• Ran bin/phd debug pulllocal --trace.
• Saw the daemon hibernate after doing a checkup on repositories.
• Saw periodic queries to look for new update messages.
• After clicking "Update Now" in the web UI to schedule an update, saw the daemon wake up immediately.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12298

Differential Revision: https://secure.phabricator.com/D17540