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.
Yes exactly. Thats why I say the use case is artificial...
But as I saw the condition, that was the easiest way to test it.