Parallelize MultipleOutputs#close call

This call takes more time when there are lot of files to close and there is a high latency to close. Parallelize MultipleOutputs#close call to improve the speed.

  public void close() throws IOException {
    for (RecordWriter writer : recordWriters.values()) {
      writer.close(null);
    }
  }

Idea is from stevel@apache.org