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.
The thread is a daemon thread, so it shouldn't hold things up.  The BlockManager heartbeat used to do pretty much the same thing.  I've tested on a cluster but didn't look at this specifically - I'll check again to make sure.