public MutableSegment createMutableSegment(final Configuration conf, CellComparator comparator,
    MemStoreSizing memstoreSizing) {
    MemStoreLAB memStoreLAB = MemStoreLAB.newInstance(conf);
    return generateMutableSegment(conf, 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