private void updateFavoredNodesForRegion(RegionInfo regionInfo, List<ServerName> newFavoredNodes)
    throws IOException {
    Map<RegionInfo, List<ServerName>> regionFNMap = Maps.newHashMap();
    regionFNMap.put(regionInfo, newFavoredNodes);
    fnm.updateFavoredNodes(regionFNMap);
  }
If we have favored nodes for a region, we will return one of the FN as destination. If favored
nodes are not present for a region, we will generate and return one of the FN as destination.
If we can't generate anything, lets fallback.
hbase-balancer/src/main/java/org/apache/hadoop/hbase/master/balancer/FavoredStochasticBalancer.java