Feature Request: iterate over a group as actors complete

`concurrent.futures` (and probably other Python stdlib stuff) has the idea of iterating over results from concurrent processing as they are completed:

https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.as_completed

I'm currently using the `Group.get_results()` method to iterate over a group of results, but it would be nice to be able to do that as the results come back instead of in the order they were queued. That way, if I have a network request hang at the very beginning, I can get other work done while that action times out and is retried.