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.
No huge reason, just that it avoids nesting the `conf.get`s and makes it easier to see what the final value is (imagine if you had 5 more gets). Though I just noticed that here you can't do that easily because of the `* 3`, so scratch that.