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.
Also, could you add a comment here that explains what this returns? I realize you added this elsewhere for `TaskScheduler*`, but it would be good to comment it here too, so people who aren't following this chain of messages have a rough idea what's going on.