BUG-1281: introduce XML event stream writer

Unlike the current DOM approach, XMLStreamWriter-driven approach allows
use to plug various backends without needing memory for intermediate
representation.

Change-Id: Ic37b570a78c681faebfcbfe573b811a121188628
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-1281: introduce XML event stream writer"