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.
Hm, this is a very odd exception. I'll try to replicate it - although I did try exactly this when testing and didn't see this same exception. How did you remove the app definition? Just by deleting the key using zkCli?