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/migrate_job.c

@@ -67,7 +67,7 @@ METHOD(job_t, destroy, void,
 	free(this);
 }
 
-METHOD(job_t, execute, void,
+METHOD(job_t, execute, job_requeue_t,
 	private_migrate_job_t *this)
 {
 	ike_sa_t *ike_sa = NULL;
@@ -117,7 +117,7 @@ METHOD(job_t, execute, void,
 	{
 		DBG1(DBG_JOB, "no CHILD_SA found with reqid {%d}", this->reqid);
 	}
-	destroy(this);
+	return JOB_REQUEUE_NONE;
 }
 
 METHOD(job_t, get_priority, job_priority_t,