private MutableSegment generateMutableSegment(final Configuration conf, CellComparator comparator,
    MemStoreLAB memStoreLAB, MemStoreSizing memstoreSizing) {
    // TBD use configuration to set type of segment
    CellSet<ExtendedCell> set = new CellSet<>(comparator);
    return new MutableSegment(set, comparator, memStoreLAB, memstoreSizing);
  }
create empty immutable segment for initializations This ImmutableSegment is used as a place
holder for snapshot in Memstore. It won't flush later, So it is not necessary to record the
initial size for it.
@param comparator comparator
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/SegmentFactory.java