BUG-2459: Fix netconf-monitoring not reusing JAXB context

JAXBContext can be freely reused, so cache it, as it has significant
footprint in testing tool.

Change-Id: If2f53b9dfd4a1d30390580fd800aa2f5eb4e2b72
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-2459: Fix netconf-monitoring not reusing JAXB context"