[SPARK-3353] parent stage should have lower stage id.

Previously parent stages had higher stage id, but parent stages are executed first. This pull request changes the behavior so parent stages would have lower stage id.

For example, command: 
```scala
sc.parallelize(1 to 10).map(x=>(x,x)).reduceByKey(_+_).count
```
breaks down into 2 stages.

The old web UI:
![screen shot 2014-09-04 at 12 42 44 am](https://cloud.githubusercontent.com/assets/323388/4146177/60fb4f42-3407-11e4-819f-853eb0e22b25.png)


Web UI with this patch:
![screen shot 2014-09-04 at 12 44 55 am](https://cloud.githubusercontent.com/assets/323388/4146178/62e08e62-3407-11e4-867b-a36b10534464.png)