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.
Readability: there are many occurrences of apps(appName). 
I think it would be more readable, if we assign to a local val for this method?