Creating mobile tombstone for existing CBS tombstone causes loop

SG 2.8.1 includes CBG-1200 to avoid creating empty document bodies when creating a mobile tombstone for a previously non-existent document.  

If the updated handling is applied to an existing, non-purged Couchbase Server tombstone that does not have a mobile xattr associated with it, Sync Gateway fails to use the cas from the existing tombstone, and so hits a cas error when writing the mobile tombstone.

The net result is Sync Gateway going into an infinite loop attempting to retry the CAS failure.