R1:9099485a7125

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…