R1:287e761f1991

Make repository synchronization safer when leaders are ambiguous

Summary:
Ref T4292. Right now, repository versions only get marked when a write happens.

This potentially creates a problem: if I pushed all the sync code to `secure` and enabled `secure002` as a repository host, the daemons would create empty copies of all the repositories on that host.

Usually, this would be fine. Most repositories have already received a write on `secure001`, so that working copy has a verison and is a leader.

However…