Update XStream drive to improve performance in xml serilization/deserialization (#2561)

* Update XStream drive to improve performance in xml serilization/deserialization

According XStream FAQ (http://x-stream.github.io/faq.html#Scalability):

XStream is a generalizing library, it inspects and handles your types on
the fly. Therefore it will normally be slower than a piece of optimized
Java code generated out of a schema. However, it is possible to increase
the performance anyway:

* Write custom converters for those of your types that occur very often in
your XML.
* Keep a configured XStream instance for multiple usage. Creation and
initialization is quite expensive compared to the overhead of XStream
when calling marshall or unmarshal.
* Use Xpp3 or StAX parsers.

So, I decided to move from old Xpp to new Xpp3.

* Change other occurance of XppDriver to Xpp3Driver as well

core/src/main/java/hudson/XmlFile.java

@@ -28,7 +28,7 @@ import com.thoughtworks.xstream.XStreamException;
 import com.thoughtworks.xstream.converters.Converter;
 import com.thoughtworks.xstream.converters.UnmarshallingContext;
 import com.thoughtworks.xstream.io.StreamException;
-import com.thoughtworks.xstream.io.xml.XppDriver;
+import com.thoughtworks.xstream.io.xml.Xpp3Driver;
 import hudson.diagnosis.OldDataMonitor;
 import hudson.model.Descriptor;
 import hudson.util.AtomicFileWriter;
@@ -292,7 +292,7 @@ public final class XmlFile {
 
     private static final SAXParserFactory JAXP = SAXParserFactory.newInstance();
 
-    private static final XppDriver DEFAULT_DRIVER = new XppDriver();
+    private static final Xpp3Driver DEFAULT_DRIVER = new Xpp3Driver();
 
     static {
         JAXP.setNamespaceAware(true);