Updated tracking / transitions of the task's version state.

[isoos]

• Moving the state transition methods into the model class.
• Retaining the docs and pana flags after scheduling (otherwise there is a time window where a new schedule has been started but not completed, where a previous run's results are not available).
• Added the previousScheduled field, but we could skip it and just use initialTimestamp when restoring the previous state.
• Simpler updatePackageStateWithPendingVersions and restorePreviousVersionsState since they no longer need to keep all the previous version state information.