Store performance for issue #358

+ Tasks are now stored as individual keys.
+ Apps using the old storage format will be migrated on their first read if found.
+ Marathon performs some cleanup of orphaned tasks once after a reboot.
+ Added comprehensive unit tests for the TaskTracker module and refactored code slightly.
I've left this as a class for now because MarathonSchedulerTest.scala mocks it when constructing MarathonScheduler. While we could use a trait, we'd lose some resolution in the test.