code: func (pool *ReaderPool) SetRate(rate int) int {
  	pool.mu.Lock()
  	old := pool.maxRate
  	pool.maxRate = rate
  	pool.setSharedRates()
  	pool.mu.Unlock()
  	return old
  }
docs: // SetRate of the pool, updating each given out reader to respect the
  // newly set rate. Returns the old rate.