private MemStoreLAB getMergedMemStoreLAB(Configuration conf, List<ImmutableSegment> segments) {
    List<MemStoreLAB> mslabs = new ArrayList<>();
    if (!conf.getBoolean(MemStoreLAB.USEMSLAB_KEY, MemStoreLAB.USEMSLAB_DEFAULT)) {
      return null;
    }
    for (ImmutableSegment segment : segments) {
      mslabs.add(segment.getMemStoreLAB());
    }
    return new ImmutableMemStoreLAB(mslabs);
  }
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