Maybe the finally callback should be removed

[Filyus]

At first glance, it is nice that you can use Promise.All and similar stuff, but this creates many independent threads that need to be processed. All you want after calling cancellation is to stop everything as possible and also stop the creation of new things like promises. I really advise you to remove this callback to avoid complicating the application logic and possible mistakes. At least write a warning in the documentation.