@Override
    public void write(SortableFeature feature) throws IOException {
      buffer.putLong(feature.key());
      buffer.putInt(feature.value().length);
      buffer.put(feature.value());
    }
Write features to the chunk file through a memory-mapped file.
planetiler-core/src/main/java/com/onthegomap/planetiler/collection/ExternalMergeSort.java