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.
Tested this:
1) Create an appdefinition
2) Stop marathon
3) remove the app definition from zookeeper
4) Start marathon
5) http :8080/v2/tasks