[SPARK-1683] Track task read metrics.

This commit adds a new metric in TaskMetrics to record
the input data size and displays this information in the UI.

An earlier version of this commit also added the read time,
which can be useful for diagnosing straggler problems,
but unfortunately that change introduced a significant performance
regression for jobs that don't do much computation. In order to
track read time, we'll need to do sampling.

The screenshots below show the UI with the new "Input" field,
which I added to the stage summary page, the executor summary page,
and the per-stage page.

![image](https://cloud.githubusercontent.com/assets/1108612/3167930/2627f92a-eb77-11e3-861c-98ea5bb7a1a2.png)

![image](https://cloud.githubusercontent.com/assets/1108612/3167936/475a889c-eb77-11e3-9706-f11c48751f17.png)

![image](https://cloud.githubusercontent.com/assets/1108612/3167948/80ebcf12-eb77-11e3-87ed-349fce6a770c.png)
There are a few issues with this.

1. This might be a bit confusing for users.
2. I think calling parallelCollectionsPartition.values will materialize the entire input partition in memory, rather than iterating through it. This could e.g. cause an OOM in cases whether otherwise the input would be streamed through.
3. The size estimation here is expensive.

So overall I'd support just removing this.