Rollback to specific version. Allow upgrades, while still in progress.

-GroupsResource: when version is given in update, rollback to given version
-GroupChange events: add version property for group events
-DeploymentPlanRepository: move to state package
-GroupManager: upgrade, while an other upgrade is in progress -> first unfinished version
-GroupRepository: load versioned entity
-DeploymentPlan: remove automatic rollback functionality
-DeploymentPlanTest: remove rollback case
-Exceptions: NoRollbackNeeded Exception no longer needed