Lazy Iterator

Is not possible now to use a lazy Iterator,
the iterator is converted to a queue https://github.com/liuggio/spawn/blob/master/src/Queue/SplQueue.php#L20 
in order to use the 'freeze' behaviour
maybe an Iterator that behave like the [QueueInterface](https://github.com/liuggio/spawn/blob/master/src/Queue) would be a better solution
