public double getAverageRegionSizeMb() {
      return averageRegionSizeMb;
    }
Inner class caries the state necessary to perform a single invocation of
{@link #computePlansForTable(TableDescriptor)}. Grabbing this data from the assignment manager
up-front allows any computed values to be realized just once.
hbase-server/src/main/java/org/apache/hadoop/hbase/master/normalizer/SimpleRegionNormalizer.java