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/libcharon/processing/jobs/send_dpd_job.c

@@ -45,7 +45,7 @@ METHOD(job_t, destroy, void,
 	free(this);
 }
 
-METHOD(job_t, execute, void,
+METHOD(job_t, execute, job_requeue_t,
 	private_send_dpd_job_t *this)
 {
 	ike_sa_t *ike_sa;
@@ -63,7 +63,7 @@ METHOD(job_t, execute, void,
 			charon->ike_sa_manager->checkin(charon->ike_sa_manager, ike_sa);
 		}
 	}
-	destroy(this);
+	return JOB_REQUEUE_NONE;
 }
 
 METHOD(job_t, get_priority, job_priority_t,