@Override
  protected Collection<StoreFileWriter> writers() {
    return lowerBoundary2Writer.values();
  }
@param lowerBoundariesPolicies each window to storage policy map.
@param needEmptyFile           whether need to create an empty store file if we haven't written
                               out anything.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/DateTieredMultiFileWriter.java