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.
should this be `shutdownNow`? My concern is that in some cases people set the akka timeout to many seconds. If the driver has stopped its responding actor, this could hang for a long time. Since this thread pool is only performing best-effort statuts updates, it seems like doing a hard shutdown might be better.