Allow overriding AsyncWorker::OnWorkComplete etc #582

As for now, AsyncWorker::OnWorkComplete and AsyncWorker::OnExecute was declared as a static private method. We could make a global static function to accept an arbitrary AsyncWorker variant and invoke its own OnWorkComplete and OnExecute to make override possible.

This would be a blocking issue to implement AsyncProgressQueueWorker, which expects a queue cleanup before invoking either OnOk and OnError in OnWorkComplete.