@Override
  public void generateFavoredNodesForMergedRegion(RegionInfo merged, RegionInfo[] mergeParents)
    throws IOException {
    updateFavoredNodesForRegion(merged, fnm.getFavoredNodes(mergeParents[0]));
  }
Generate favored nodes for a region during merge. Choose the FN from one of the sources to keep
it simple.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/FavoredStochasticBalancer.java