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.
You should `getOrElseUpdate` here. It is possible for events to be dropped from the queue such that we could receive an event for a task we don't know about. In that case, it's best to deal with it as normal and log a warning.