code: public void fsync(long sequence, K key, Result<Boolean> result)
    {
      scheduleFsync(sequence, key, result);
      flush();
    }
docs: Request an fsync for the allocated sequence, and request a flush
  to occur at the next service batch.
  
  @param sequence the sequence id allocated from allocateFsyncSequence()
  @param result the completion to be called after the fsync completes