XDCR - replication protection when running in mixed mode

When a cluster is running in mixed mode where one node is at a legacy version and another node is in a newer version, XDCR does not check the source cluster's clusterCompatibility before allowing replications to be created/edited. This can lead to situations as MB-45926.

XDCR backend should be more intelligent and handle mixed mode in a way that is future proof, as more and more features are added in future releases to prevent unexpected and undefined behavior.