public ImmutableSegment createImmutableSegment(MutableSegment segment,
    MemStoreSizing memstoreSizing) {
    return new CSLMImmutableSegment(segment, 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