Don't propagate pause/resume/cancel to upstream.

[inamiy]

This pull request will fix super weird misinterpretation in #10.

For example,

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

when task3.pause() is invoked,

As-Is (from #10)

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

To-Be

• DON'T EVER CALL task1.pause() because task1 should be totally independent from task3, as task1 is capable of multiple-then() handling (let's say task3a, task3b, task3c is created via multiple task1.then(), but no one want task3a.cancel() to fail task1 and thus delivering rejected-errorInfo to task3b & task3c).
• only task2.pause() if task1 is either fulfilled or rejected & task2 is still running