code: func PrefetchSize(size int) Option {
  	return func(r *Rabbus) error {
  		r.config.qos.prefetchSize = size
  		return nil
  	}
  }
docs: // PrefetchSize when greater than zero, the server will try to keep at least
  // that many bytes of deliveries flushed to the network before receiving
  // acknowledgments from the consumers.