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/delete_ike_sa_job.c

@@ -48,7 +48,7 @@ METHOD(job_t, destroy, void,
 	free(this);
 }
 
-METHOD(job_t, execute, void,
+METHOD(job_t, execute, job_requeue_t,
 	private_delete_ike_sa_job_t *this)
 {
 	ike_sa_t *ike_sa;
@@ -60,7 +60,7 @@ METHOD(job_t, execute, void,
 		if (ike_sa->get_state(ike_sa) == IKE_PASSIVE)
 		{
 			charon->ike_sa_manager->checkin(charon->ike_sa_manager, ike_sa);
-			return destroy(this);
+			return JOB_REQUEUE_NONE;
 		}
 		if (this->delete_if_established)
 		{
@@ -89,7 +89,7 @@ METHOD(job_t, execute, void,
 			}
 		}
 	}
-	destroy(this);
+	return JOB_REQUEUE_NONE;
 }
 
 METHOD(job_t, get_priority, job_priority_t,