token_bucket.go
func (tb *tokenBucket) capacityToken() time.Time {
	return time.Now().Add(-tb.refillDuration).Truncate(tb.tokenInterval)
}
func (tb *tokenBucket) capacityToken() time.Time {
	return time.Now().Add(-tb.refillDuration).Truncate(tb.tokenInterval)
}
// Determine the historic token timestamp representing a full bucket