Replace the xpp pull parser in xstream with a java6+ standard solution

**benoit.wiart** ([Bug 58679](https://bz.apache.org/bugzilla//show_bug.cgi?id=58679&redirect=false)):
in current jmeter version xstream use the xpp library to provide the xml pull parser functionality.
Since Java6 it's possible to use the xstream staxDriver which does not need the xpp library

See http://x-stream.github.io/tutorial.html#init

PR will follow.

Testing was done with the jmeter test suite + local tests

Severity: major
OS: All

Blocks:
* https://github.com/apache/jmeter/issues/4056