Give processor_t more control over the lifecycle of a job

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.

src/libstrongswan/processing/scheduler.c

@@ -250,6 +250,7 @@ METHOD(scheduler_t, schedule_job_tv, void,
 
 	event = malloc_thing(event_t);
 	event->job = job;
+	event->job->status = JOB_STATUS_QUEUED;
 	event->time = tv;
 
 	this->mutex->lock(this->mutex);