backoff.go
func NewExponentialFullJitter(start time.Duration, limit time.Duration) *Backoff {
	return NewBackoff(exponentialFullJitter{limit: limit}, start, limit)
}
func NewExponentialFullJitter(start time.Duration, limit time.Duration) *Backoff {
	return NewBackoff(exponentialFullJitter{limit: limit}, start, limit)
}
// NewExponentialFullJitter creates a new backoff using the exponential with full jitter backoff algorithm.