Change Packet.getExtensionsXML()

Remove the synchronized, as getExtensions() is properly synchronized and will
return a copy.

Return XmlStringBuilder instead of CharSequence to take advantage of
fast XmlStringBuilder.append(XmlStringBuilder).

Mark the method as final, as it should not be overwritten.

smack-core/src/main/java/org/jivesoftware/smack/packet/Packet.java

@@ -354,7 +354,7 @@ public abstract class Packet extends TopLevelStreamElement {
      * @return the extension sub-packets as XML or the Empty String if there
      * are no packet extensions.
      */
-    protected synchronized CharSequence getExtensionsXML() {
+    protected final XmlStringBuilder getExtensionsXML() {
         XmlStringBuilder xml = new XmlStringBuilder();
         // Add in all standard extension sub-packets.
         for (PacketExtension extension : getExtensions()) {