Fix hazelcast plugin xalan NoClassDefFoundError

	issue:hazelcast/hazelcast#6614

src/plugins/hazelcast/src/java/org/jivesoftware/openfire/plugin/HazelcastPlugin.java

@@ -48,6 +48,9 @@ public class HazelcastPlugin extends TimerTask implements Plugin {
     private static final long CLUSTER_STARTUP_DELAY_TIME = 
     		JiveGlobals.getLongProperty("hazelcast.startup.delay.seconds", 5);
 
+    private static final String TRANSFORMERFACTORY = "javax.xml.transform.TransformerFactory";
+    private static String OLDTRANSFORMERFACTORY;
+
     public void initializePlugin(PluginManager manager, File pluginDirectory) {
     	// start cluster using a separate thread after a short delay
     	// this will allow other plugins to initialize during startup
@@ -72,10 +75,16 @@ public boolean accept(File pathname) {
             logger.warn("Conflicting clustering plugins found; remove Coherence and/or Enterprise jar files");
             throw new IllegalStateException("Clustering plugin configuration conflict (Coherence)");
         }
+
+        // https://github.com/hazelcast/hazelcast/issues/6614
+        OLDTRANSFORMERFACTORY = System.getProperty(TRANSFORMERFACTORY);
+        System.setProperty(TRANSFORMERFACTORY, "com.sun.org.apache.xalan.internal.xsltc.trax.TransformerFactoryImpl");
+
         ClusterManager.startup();
 	}
 
     public void destroyPlugin() {
+    	System.setProperty(TRANSFORMERFACTORY, OLDTRANSFORMERFACTORY);
         // Shutdown is initiated by XMPPServer before unloading plugins
     	if (!XMPPServer.getInstance().isShuttingDown()) {
     		ClusterManager.shutdown();