Run selfplay and write a training chunk to the fsdb golden_chunk_dir.

  Args:
    state: the RL loop State instance.
    flagfile: the name of the flagfile to use for selfplay, either 'selfplay'
        (the default) or 'boostrap'.

async def selfplay(state, flagfile='selfplay'):
  """Run selfplay and write a training chunk to the fsdb golden_chunk_dir.

  Args:
    state: the RL loop State instance.
    flagfile: the name of the flagfile to use for selfplay, either 'selfplay'
        (the default) or 'boostrap'.
  """

  output_dir = os.path.join(fsdb.selfplay_dir(), state.output_model_name)
  holdout_dir = os.path.join(fsdb.holdout_dir(), state.output_model_name)

  lines = await run(
      'bazel-bin/cc/selfplay',
      '--flagfile={}.flags'.format(os.path.join(FLAGS.flags_dir, flagfile)),
      '--model={}'.format(state.best_model_path),
      '--output_dir={}'.format(output_dir),
      '--holdout_dir={}'.format(holdout_dir),
      '--seed={}'.format(state.seed))
  result = '\n'.join(lines[-6:])
  logging.info(result)
  stats = parse_win_stats_table(result, 1)[0]
  num_games = stats.total_wins
  logging.info('Black won %0.3f, white won %0.3f',
               stats.black_wins.total / num_games,
               stats.white_wins.total / num_games)

  # Write examples to a single record.
  pattern = os.path.join(output_dir, '*', '*.zz')
  random.seed(state.seed)
  tf.set_random_seed(state.seed)
  np.random.seed(state.seed)
  # TODO(tommadams): This method of generating one golden chunk per generation
  # is sub-optimal because each chunk gets reused multiple times for training,
  # introducing bias. Instead, a fresh dataset should be uniformly sampled out
  # of *all* games in the training window before the start of each training run.
  buffer = example_buffer.ExampleBuffer(sampling_frac=1.0)

  # TODO(tommadams): parallel_fill is currently non-deterministic. Make it not
  # so.
  logging.info('Writing golden chunk from "{}"'.format(pattern))
  buffer.parallel_fill(tf.gfile.Glob(pattern))
  buffer.flush(os.path.join(fsdb.golden_chunk_dir(),
                            state.output_model_name + '.tfrecord.zz'))