Refactor Status Transition Logic

[sivkovic]

Move logic for what happens when a status is changed to the model. E.g. if a Job goes from RUNNING to COMPLETED, we can update the finished_date in the save function similar to how it's done in Beagle -- so the business logic in the check_status_of_jobs task is easier to read.
- Status should only be updated in a single place - remove all status changes from all other methods
- FAILED jobs we'd just have the decorator catch exceptions
- Remove functions with side effects like update_message