@Override
    public void write(SortableFeature feature) throws IOException {
      out.writeLong(feature.key());
      out.writeInt(feature.value().length);
      out.write(feature.value());
    }
Write features to the chunk file using a {@link BufferedOutputStream}.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java