BUG/MINOR: resolvers: fix missing task_idle destruction in resolvers_destroy()

When destroying a stream-based DNS nameserver, task_req and task_rsp
were destroyed but task_idle was missed, causing a task object leak.
This doesn't necessarily have to be backported since it's only upon
exit that it is visible.

src/resolvers.c

@@ -2696,6 +2696,7 @@ static void resolvers_destroy(struct resolvers *resolvers)
 			dns_ring_free(ns->stream->ring_req);
 			task_destroy(ns->stream->task_req);
 			task_destroy(ns->stream->task_rsp);
+			task_destroy(ns->stream->task_idle);
 			free(ns->stream);
 		}
 		LIST_DEL_INIT(&ns->list);