Range scanner which iterates over ranges and adds them to queues.

Next step will be implementation of rangeIterator, integration with
Store, and creation of first actual rangeQueue implementation. Will
probably do garbage collection.
It's weird that we sleep in between advancing the iterator and adding the resulting range to the queue. I think I would put this logic at the top of the for loop before the select, so the body of the `case <-time.After()` is empty (this would go a little faster than the designated pace when remove is called, but that's fine).