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.
Can we rename this to something more explicit, something like `executorHeartbeatReceived`? This is also more consistent with other methods in this actor (e.g. `executorLost`, `taskEnded`). From the name itself it's not super clear whether we're sending or receiving the heartbeat.