SPARK-2099. Report progress while task is running.

This is a sketch of a patch that allows the UI to show metrics for tasks that have not yet completed.  It adds a heartbeat every 2 seconds from the executors to the driver, reporting metrics for all of the executor's tasks. 

It still needs unit tests, polish, and cluster testing, but I wanted to put it up to get feedback on the approach.
I think this is a little hard to read right now. Could you do the following here?
```
taskMetrics.flatMap { case (id, metrics) =>
  ..
}
```