add fallback_server so we can still have a different default server on first join

api/src/main/java/net/md_5/bungee/api/config/ListenerInfo.java

@@ -32,6 +32,10 @@ public class ListenerInfo
      * Name of the server which users will be taken to by default.
      */
     private final String defaultServer;
+    /**
+     * Name of the server which users will be taken when current server goes down.
+     */
+    private final String fallbackServer;
     /**
      * Whether reconnect locations will be used, or else the user is simply
      * transferred to the default server on connect.