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.
We should mark this @DeveloperApi like the other events. Also, same formatting nit:
```
case class SparkListenerExecutorMetricsUpdate(
    execId: String,
    taskMetrics: Seq[(Long, Int, TaskMetrics)])
  extends SparkListenerEvent
```