mxqd: Sleep, when external helper failed to set up tmpdir

Throttle main loop in case this is a systematic error.

mxqd.c

@@ -1323,6 +1323,7 @@ static unsigned long start_job(struct mxq_group_list *glist)
         if (status == -1) {
             mx_log_err("create job tmpdir: %m");
             mxq_unload_job_from_server(server->mysql, job->job_id);
+            sleep(30);
             return 0;
         }
     }