[ADD] new XMLRPC endpoint POC

* uses new calling conventions
* serializes recordsets
* less idiosyncratic

Custom xmlrpc marshaller:

Turns out the existing xmlrpclib.Marshaller isn't really extensible, and
trying to add dumping for recordsets (-> [ids]) didn't work.

So create a custom marshaller generating an ElementTree dict. Should
probably use a xml.sax.saxutils.XMLGenerator instead, except I don't
think we can get things right wrt serialization error (e.g. None), that
would require interrupting the output stream and sending a new
response...

TODO: serializing a correct Fault when the response serialization fails.