Handlers can now modify Datasets on-the-fly and influence the
structure of the generated backup jobs. Backup job creation is also
delegated to Handlers.

The Job and Scheduler objects are fairly generic.

To avoid conflicts with the job manager status type.

It must be at least version 0.9 in order to support the expected
restore semantics.

Use a configuration manager to handle runtime config-derived data and
eventually allow live configuration reloading.