R1:7dd55a728f1e

Make repository daemons periodically check for out-of-sync repositories

Summary:
See PHI1015. If you add new repository nodes to a cluster, we may not actually sync some repositories for up to 6 hours (if they've had no commits for 30 days).

Add an explicit check for out-of-sync repositories to trigger background sync.

Test Plan:
- Ran `bin/phd debug pullocal`.
- Fiddled with the `repository_workingcopy` version table to put the local node in and out of sync with the cluster.
- Saw appropriate…