Add "as completed" iterator

[bergundy]

Allow streaming progress from multiple async operations

https://docs.python.org/3/library/concurrent.futures.html#threadpoolexecutor-example
https://docs.python.org/3/library/concurrent.futures.html#concurrent.futures.as_completed
http://www.tornadoweb.org/en/stable/gen.html#tornado.gen.WaitIterator