[BP 7.2.5] - XDCR - potentially skip rollback to 0 if purge seqno moved

DCP documentation states Wildcard purge: https://github.com/couchbase/kv_engine/blob/master/docs/dcp/documentation/rollback.md#2-wild-card-purge
as:

This is necessary because for a consistent view the consumer should not miss out on any deleted (and subsequently purged items) on the producer.

XDCR as a DCP client is a bit of an oddity, simply because while it utilizes the DCP protocol, the target bucket is not going to represent a consistent view of the source bucket, because XDCR does not enforce a target bucket to perform any rollbacks.

As such, in this specific case where DCP expects the client to rollback to 0 so that the client can have a consistent view of the database, it isn't true because those documents that have been purged will still remain on the target bucket (as XDCR doesn't flush the target bucket).

This MB is to mark a task for the appropriate next release after 7.0 to visit this situation. In essence, we should look into whether or not it is possible for XDCR to ask DCP to violate this "consistent view" requirement specifically for the purge seqno case (i.e. via a flag during streamReq). By enforcing a rollback to 0 adds no value for this case and CBSEs have been raised due to rollback.