Feature request: PathMapper trickle iteration

Depends on #5

Often, the items from a `PathMapper` are fed into a `concurrent.futures.ThreadPoolExecutor`. We should think of a way of how we can throttle the iteration, to stay in a safe zone under the memory limit.

Pseudocode:

1. yield first item
2. while not empty...
3. sleep 5 seconds
4. how much did memory usage increase? if available memory is greater than average memory increase per item, then yield next item.