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.
If we have 100 executors with 24 cores and a heartbeat interval of 2 seconds, we'll end up with 1200 objects per second.  That doesn't seem enormous to me, but also does seem like unnecessary overhead.  Are there hidden costs in Scala to keeping this mutable?