Return error when receiving deltas for deltaSrc revisions which are tombstoned

Sync Gateway will attempt to receive and patch a delta for a document which has a parent/deltaSrc revision that has been tombstoned.

This is a situation that SG can't deal with, as the parent body is not available to Sync Gateway when it is tombstoned when running with xattrs.

A mitigation for this went into CBL for Cobalt, but SG should still be defensive about this and return an error for a client that pushes a delta for a tombstone. This may need some co-ordination with the Lite team to make sure the error is handled correctly if this situation ever arises on their side.