async_map: fix bug

[oliver-sanders]

Fix a really embarrassing bug in the async_map function (which is used for the --ordered option to cylc scan) 😢 .

[kinow]

Leaving for @wxtim the merge and final review 👍

[matthewrmshin]

Not part of this change, but doc string at line 43 has duplicated iterator.

[oliver-sanders]

Think I cut off the last two lines when I copied the code...

[wxtim]

Looks sensible to me.

[dwsutherland]

Hmmm.. How does changed ever be anything but True?

        changed = True
        while changed and completed_tasks:
            if index in completed_tasks:
                yield completed_tasks.pop(index)
                changed = True
                index += 1
            else:
                break

[oliver-sanders]

Hmmm.. How does changed ever be anything but True?

Dammit.