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 responses in the daemon (sync; wait if the last sync trigger was too recent).

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D20078