R: sync client bailing out

[F. Rossetti]

>> Now everything seems to have come back up but the replication still has 
>> problems. The client process on the master bails out frequently with 
>> these errors in the log:
>
>It looks like you have some corruption in cyrus.index files on the replica 
>following the crash. Try running reconstruct on the mailboxes in question.

Some corruption indeed: the replica had faulty RAM. I ended up wiping out
the mailboxes on the replica and started everything fresh, the data was
mostly corrupted anyway. One should run memtest86 every once in a while..