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.
Looks like `HeartbeatReceiver` can only exist on the driver (because it tasked in a scheduler). Then I don't see why we can't just call `blockManagerMaster.heartBeat(...)` (and we should rename this from `heartBeat`) directly rather than having to go through another akka exchange.