allow optimizer to be stopped from outside

[lelandjobson]

• Move async fuctionality to inside of optimizer
• Method that raises a flag to stop optimizer on next iteration

[EmilPoulsen]

Yeah, that makes sense. Basically start the optimization from inside the optimizer on a separate thread and then raise events on completion etc.

That way we should be able to stop the process respectfully from outside, without having to kill the whole thread