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

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