Add TimeoutToken

[bergus]

A token that is cancelled after a timeout is simple:

new CancelToken(cancel => setTimeout(cancel, t, "timeout"))

However, this keeps the setTimeout active even when the token is no longer needed and all promises that might be cancelled by it are already resolved. This could keep a process alive even when there's nothing to do any more.
Implement a TimeoutToken that does not need any resources while nobody is listening, i.e. simply stores the end time and calls setTimeout/clearTimeout as necessary.
In node, this also could be achieved by a simple unref call.