BUG-1281: rework XmlStreamUtils a bit more

This adds documentation to XmlStreamUtils and reworks the structure to
properly encapsulate the XmlCodecProvider.

Change-Id: I038f86b95f4205d452860fad9c9e38af61f48a2a
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1281: rework XmlStreamUtils a bit more"