Domain bucket tombstones

Fixed tombstone handling in DomainBucket
    
DomainBucket.fetch() was not honoring the ReturnDeletedVClock
setting in the FetchMeta.
    
Further investigation found that there was no way to perform a delete
via DomainBucket using a vclock and a bug set the option for the Delete to
fetch the vclock to false.
    
DomainBucket.delete() now will either take a vclock or tell the
delete operation to fetch it first if it's not present.

A minor optimization was added to DeleteObject where
when fetching the vclock if there's only a tombstone it just
returns rather than performing a delete.