private void rollWriters(WriterLength writerLength) throws IOException {
        if (writerLength != null) {
          closeWriter(writerLength);
        } else {
          for (WriterLength wl : this.writers.values()) {
            closeWriter(wl);
          }
        }
      }
ExtendedCell and ExtendedCellSerialization are InterfaceAudience.Private. We expose this config
for internal usage in jobs like WALPlayer which need to use features of ExtendedCell.
hbase-mapreduce/src/main/java/org/apache/hadoop/hbase/mapreduce/HFileOutputFormat2.java