userdbd: drastically raise ratelimit we apply on requests for more worker processes

These requests might come in during lookup floods very quickly, since
multiple worker processes might detect that things should be scaled up
at the same time. Hence, let's substantially raise the limit so that it
doesn't get hit in real-life scenarios and acts more like a safety net.

src/userdb/userdbd-manager.c

@@ -71,8 +71,8 @@ int manager_new(Manager **ret) {
         *m = (Manager) {
                 .listen_fd = -EBADF,
                 .worker_ratelimit = {
-                        .interval = 5 * USEC_PER_SEC,
-                        .burst = 50,
+                        .interval = 2 * USEC_PER_SEC,
+                        .burst = 2500,
                 },
         };