yang-data-codec-xml: reuse codecs across documents

XmlCodecFactory is making the mistake of caching the codecs based on the combination of NamespaceContext and TypeDefinition.

This means the codecs cannot be reused across documents, as these result in distinct NamespaceContext.

The XmlCodec interface already provides a writer, from which we can get the namespace context, hence it should be relatively easy to pass this down for the duration of the serialization request.