@Override
  protected SortedSet<ExtendedCell> tailSet(ExtendedCell firstCell) {
    throw new IllegalStateException("Not supported by CompositeImmutableScanner");
  }
Returns a subset of the segment cell set, which starts with the given cell
@param firstCell a cell in the segment
@return a subset of the segment cell set, which starts with the given cell
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompositeImmutableSegment.java