This ensures the NM-specific credential set is unloaded before any
implementation of certificate/key objects, which causes a segmentation
fault during shutdown.
This ensures that no threads are active when plugins and the rest of the
daemon are unloaded.
callback_job_t was simplified a lot in the process as its main
functionality is now contained in processor_t. The parent-child
relationships were abandoned as these were only needed to simplify job
cancellation.