Added mandatory property in latest GoogleAppEngine versions

Author: ctasada

gwt-cal-demo/war/WEB-INF/appengine-web.xml

@@ -3,6 +3,12 @@
 	<application>gwt-web-calendar</application>
 	<version>google</version>
 
+  <!--
+    By default, App Engine sends requests serially to a given web server.
+    To allow App Engine to send multiple requests in parallel specify:
+   -->
+   <threadsafe>true</threadsafe>
+   
 	<!-- Configure java.util.logging -->
 	<system-properties>
 		<property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>