Preserve streaming behavior in camel-cxf PAYLOAD mode

Dan Kulp pointed out in https://issues.apache.org/jira/browse/CXF-2753 that Camel should preserve the streaming behavior by setting message part type class to Source.class (so the payload object becomes a SAXSource that wraps around a XMLStreamReader). The camel-cxf component currently exposes to the user a List of DOM Elements as payload. Basically, streaming behavior stops there anyway. We probably should provide a parameter to configure the camel-cxf endpoint to opt out of DOM and get a SAXSource as payload and add a converter if the user needs to go from SAXSource to DOM.