defaultBackoffStrategy function
Implementation
Duration defaultBackoffStrategy(Duration? lastBackoff) {
if (lastBackoff == null) return _initialBackoff;
final jitter = _random.nextDouble() * 2 * _jitter - _jitter;
final nextBackoff = lastBackoff * (_multiplier + jitter);
return nextBackoff < _maxBackoff ? nextBackoff : _maxBackoff;
}