Avoid performance issue on SAXParserFactory retrieval

Executing SAXParserFactory.newInstance() on WildFly results in few exceptions being thrown and caught to eventually figure out the proper SAXParserFactory for the current ThreadContext classloader; this badly affects performances of the RESTEasy JAXB provider.
We could mitigate the issue by having a caching mechanism of SAXParserFactory instances in the org.jboss.resteasy.plugins.providers.jaxb.SecureUnmarshaller.