keep the order of the callbacks

[narimiran]

Put new callbacks at the end of the CallbackList, not in front, so the order of callbacks is not reversed.

Fixes #7197.

[krux02]

I just want to mention that atexit reverse the order of callbacks for good reasons. They reverse it, because they are often used to register destructors in C, and here it is important that the destruction is done in reverse order.

[dom96]

Honestly I'm not sure why it's such a big deal that the callbacks were reversed, but shrug. I didn't want multiple callbacks on futures in the first place.

[Araq]

here it is important that the destruction is done in reverse order.

Not really, but it also doesn't hurt. Quick analysis:

Destruction calls dealloc. Order irrelevant.
Destruction calls close on a handle. Order irrelevant.
Destruction calls release on a lock. Order relevant but omg, who does that?!

[krux02]

well the order is relevant when the destructor wants to close a file that is stored in a member of an object that has already been deallocated.

[Araq]

That would have been a single atexit handler then, not 2. Assuming somewhat sane programming practices.