dsync: If incremental dsync finds that its state is stale, retry (later) with full...

dsync: If incremental dsync finds that its state is stale, retry (later) with full sync.
This works only when the master itself notices such changes locally. Remote
changes aren't noticed, because the master doesn't send the state to remote
dsync and the master is also the one that decides which mailboxes are
synced. The fix to this probably means sending the state string to remote
dsync and have it check if the state is still valid (= protocol change).