@Override
  public void setRSGroup(Set<TableName> tables, String groupName) throws IOException {
    getRSGroupInfo(groupName);
    moveTablesAndWait(tables, groupName);
  }
This is an EXPENSIVE clone. Cloning though is the safest thing to do. Can't let out original
since it can change and at least the load balancer wants to iterate this exported list. Load
balancer should iterate over this list because cloned list will ignore disabled table and split
parent region cases. This method is invoked by {@link #balanceRSGroup}
@return A clone of current assignments for this group.
hbase-server/src/main/java/org/apache/hadoop/hbase/rsgroup/RSGroupInfoManagerImpl.java