[SPARK-3984] [SPARK-3983] Improve UI task metrics.

This commit fixes the scheduler delay in the UI (which previously
included things that are not scheduler delay, like time to
deserialize the task and serialize the result), and also
adds finer-grained information to the summary table for each
stage about task launch overhead (which is useful for debugging
performance of short jobs, where the overhead is not-insignificant).

What the new summary metrics table looks like (I also rearranged the 
fields slightly, so they appear in roughly chronological order of what
happens for each task):
![image](https://cloud.githubusercontent.com/assets/1108612/4672978/42009f98-559a-11e4-9230-d2bb9c19bdf1.png)

cc @shivaram @etrain