OF-1147: Thread safety when modifying admin console

When there are multiple plugins loading at the same time, they can
concurrently attempt to add models to the admin console. To prevent
concurrency issues, those methods are made synchronous.

src/java/org/jivesoftware/admin/AdminConsole.java

@@ -87,7 +87,7 @@ public class AdminConsole {
      * @param element the Element
      * @throws Exception if an error occurs.
      */
-    public static void addModel(String name, Element element) throws Exception {
+    public static synchronized void addModel(String name, Element element) throws Exception {
         overrideModels.put(name, element);
         rebuildModel();
     }
@@ -97,7 +97,7 @@ public class AdminConsole {
      *
      * @param name the name.
      */
-    public static void removeModel(String name) {
+    public static synchronized void removeModel(String name) {
         overrideModels.remove(name);
         rebuildModel();
     }