LWW for delete or json document in custom Bucket needs more than CAS for conflict resolution

For custom bucket, XDCR fetch target XATTRs using subdoc_multi_lookup, which only returns Cas and not revId, flags, etc. For non-json or deleted doc, we currently use LWW based on Cas only. This is different from LWW bucket. It is possible for different documents to have the same Cas but otherwise different and these documents will not converge. Need to take another look at this.