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.
Jobs are now destroyed by the processor, but they are allowed to
reschedule themselves. That is, parts of the reschedule functionality
already provided by callback_job_t is moved to the processor. Not yet
fully supported is JOB_REQUEUE_DIRECT and canceling jobs.
Note: job_t.destroy() is now called not only for queued jobs but also
after execution or cancellation of jobs. job_t.status can be used to
decide what to do in said method.
Andreas Steffen