token_bucket.go
func newTokenBucket(capacity int64, tokenInterval time.Duration) *tokenBucket {
	tb := tokenBucket{
		capacity:       capacity,
		tokenInterval:  tokenInterval,
		refillDuration: tokenInterval * time.Duration(capacity)}

	tb.earliestUnspentToken = tb.capacityToken()

	return &tb
}
func newTokenBucket(capacity int64, tokenInterval time.Duration) *tokenBucket {
	tb := tokenBucket{
		capacity:       capacity,
		tokenInterval:  tokenInterval,
		refillDuration: tokenInterval * time.Duration(capacity)}

	tb.earliestUnspentToken = tb.capacityToken()

	return &tb
}
// newTokenBucket returns a bucket containing capacity tokens, refilled at a
// rate of one token per tokenInterval.