Fix bug of not destroying threads that monitor stale broadcasts.

src/altk/comm/engine/CommEngine.java

@@ -469,6 +469,9 @@ public abstract class CommEngine extends HttpServlet
     {
         System.out.println(engineName + " destroyed");
         
+        // Shutdown threads that periodically purge stale broadcasts.
+        scheduler.shutdownNow();
+        
         // Kill threads in each PostBack, which is remembered in postBackMap.
         for (PostBack postback : postBackMap.values())
         {
@@ -481,6 +484,7 @@ public abstract class CommEngine extends HttpServlet
         }
         
         destroyChild();
+        super.destroy();
     }
 
     /**