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.
As discussed offline, I didn't realize the `heartBeat` method was part of the actor, but not the `BlockManagerMaster` itself. Then it's perfectly reasonable to use akka for this.