@Override
  public ExtendedCell maybeCloneWithAllocator(ExtendedCell cell, boolean forceCloneOfBigCell) {
    throw new IllegalStateException("Not supported by CompositeImmutableScanner");
  }
If the segment has a memory allocator the cell is being cloned to this space, and returned;
otherwise the given cell is returned
@return either the given cell or its clone
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/CompositeImmutableSegment.java