code: KeyValue compact(final ArrayList<KeyValue> row,
        List<Annotation> annotations,
        List<HistogramDataPoint> histograms) {
      final KeyValue[] compacted = { null };
      compact(row, compacted, annotations, histograms);
      return compacted[0];
    }
docs: Compacts a row into a single {@link KeyValue}.
  @param row The row containing all the KVs to compact.
  Must contain at least one element.
  @return A compacted version of this row.