only run pre_callbacks if the caller tells us to wait

only the network & worker threads use the pre callbacks.  And,
the only thing they do is lower the wait time.  So if we're told
to not wait, then don't bother asking caller if the timers
should be lowered.

src/lib/util/event.c

@@ -1478,13 +1478,18 @@ int fr_event_corral(fr_event_list_t *el, fr_time_t now, bool wait)
 	 *	Run the status callbacks.  It may tell us that the
 	 *	application has more work to do, in which case we
 	 *	re-set the timeout to be instant.
+	 *
+	 *	We only run these callbacks if the caller is otherwise
+	 *	idle.
 	 */
-	for (pre = fr_dlist_head(&el->pre_callbacks);
-	     pre != NULL;
-	     pre = fr_dlist_next(&el->pre_callbacks, pre)) {
-		if (pre->callback(pre->uctx, wake ? *wake : 0) > 0) {
-			wake = &when;
-			when = 0;
+	if (wait) {
+		for (pre = fr_dlist_head(&el->pre_callbacks);
+		     pre != NULL;
+		     pre = fr_dlist_next(&el->pre_callbacks, pre)) {
+			if (pre->callback(pre->uctx, wake ? *wake : 0) > 0) {
+				wake = &when;
+				when = 0;
+			}
 		}
 	}