@Override
    public KeyValue deserialize(ExtendedCell ignore) throws IOException {
      KeyValue kv = KeyValueUtil.create(this.dis);
      PrivateCellUtil.setSequenceId(kv, this.dis.readLong());
      return kv;
    }
Similar to CellSerialization, but includes the sequenceId from an ExtendedCell. This is necessary
so that CellSortReducer can sort by sequenceId, if applicable. Note that these two serializations
are not compatible -- data serialized by CellSerialization cannot be deserialized with
ExtendedCellSerialization and vice versa. This is ok for {@link HFileOutputFormat2} because the
serialization is not actually used for the actual written HFiles, just intermediate data (between
mapper and reducer of a single job).
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/ExtendedCellSerialization.java