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 origin of this is still from executors.  The same two purposes remain - let the blockmanager master know that the executor's is still alive and find out whether the executor's blockmanager should reregister.  It just gets passed along inside the generic executor->driver instead of as its own message.