Better progress handling than giving it to thread 0

[djsutherland]

Sometimes thread 0 finishes earlier than the stragglers, and the progressbar hangs at the end.

Instead, ideally each thread would say "hey, is the GIL available to take?" – if so, it'd grab it and update the progressbar, but if not, it'd just continue on its way.

Have to look into whether this is possible.