@Override
    public boolean hasNext() {
      return descending ? (index >= minCellIdx) : (index < maxCellIdx);
    }
Get the index of the given anchor key for creating subsequent set. It doesn't matter whether
the given key exists in the set or not. taking into consideration whether the key should be
inclusive or exclusive.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CellFlatMap.java