replication is unable to fix uidvalidity difference

[Bron Gondwana]

Our "checkreplication" script has managed to find uidvalidity
differences between a couple of folders.  I'm not sure how they
crept in, probably other bugs causing reconstructs with missing
index files or something.  Anyway...

Reading the sync_client protocol, I notice that do_mailbox_single
doesn't actually send the uidvalidity as part of the local mailbox
status from the server, so it can't even tell there's a difference.

The clients will get very confused though, especially if switching
from a higher to a lower uidvalidity after a failover.

I'm loathe to push a sync protocol change in since we can fix easily
enough by issuing a delete against the mailbox on the replica and
re-syncing by hand.  It's pretty rare.

Still, something to keep in mind next time there's a protocol change.
It wouldn't be much work to add.

Bron.