Generic operation tasks should be able to return specific results

The generic interface for operation management tasks added with OAK-1160 does so far not provide a way for specific tasks to return a value apart from a genetic status. With the consistency checking we are starting to add (OAK-1448) such a needs start to arise.

To address this I propose to change type of the tasks that can be passed to the constructor of ManagementOperation. The result type of the task (i.e. Callable) should change from Long to some generic container, which would carry the result of the task.