[SPARK-4834] [standalone] Clean up application files after app finishes.

Commit 7aacb7bfa added support for sharing downloaded files among multiple
executors of the same app. That works great in Yarn, since the app's directory
is cleaned up after the app is done.

But Spark standalone mode didn't do that, so the lock/cache files created
by that change were left around and could eventually fill up the disk hosting
/tmp.

To solve that, create app-specific directories under the local dirs when
launching executors. Multiple executors launched by the same Worker will
use the same app directories, so they should be able to share the downloaded
files. When the application finishes, a new message is sent to all workers
telling them the application has finished; once that message has been received,
and all executors registered for the application shut down, then those
directories will be cleaned up by the Worker.

Note 1: Unit testing this is hard (if even possible), since local-cluster mode
doesn't seem to leave the Master/Worker daemons running long enough after
`sc.stop()` is called for the clean up protocol to take effect.

Note 2: the code tracking finished apps / app directories in Master.scala
and Worker.scala is not really thread-safe, but then the code that modifies
other shared maps in those classes isn't either, so this change is not making
anything worse.
Minor nit, but I think it would be clearer to introduce an `appDirectory` variable in the outer `foreach` rather than using the underscore here.