Enhance balancer to make sure no meta primary/replica regions are going to be assigned to one same region server.

Region replica has enhancement in balancer that primary region and its replicas are not going to be assigned to the same region server. Today, there is only one meta region, so this enhancement is still enough. With split meta coming in, it needs to make sure that no meta regoin/replicas is going to be assigned to the same region server in order to avoid hotspot issue.