Fix pause/cancel bug when chained with then/success/catch.

[inamiy]

This pull request will allow chained task to pause/cancel its parent task.

For example,

let task3 = task1.then { ...
    return task2
}

task3.pause() will invoke:

• task1.pause() if task1 is still running
• task2.pause() if task1 is either fulfilled or rejected & task2 is still running

and so will cancel().

FYI, bug 568e012 was discovered when I was adding new retryable feature try() in 9438564 #9.