kamailio.cfg: use t_precheck_trans() to catch retransmissions faster

etc/kamailio.cfg

@@ -474,6 +474,11 @@ request_route {
 
 	### only initial requests (no To tag)
 
+	# handle retransmissions
+	if(t_precheck_trans()) {
+		t_check_trans();
+		exit;
+	}
 	t_check_trans();
 
 	# authentication