[FIXED JENKINS-12542] upgraded to XStream 1.4.4

There are several incompatibilities in XStream 1.4 that resulted in our
changing the core, but according to the text search on all the public
plugins, none of these changes affect the 600+ plugins we have today.

However, it is still possible that there are some parts of XStream 1.3
that plugins depend on but the core doesn't depend on --- we won't be
able to tell those problems at this point.