Incorrect handling of filter namespaces

Current implementation of RESTCONF filters does not handle namespaces correctly, as it performs a namespace-ignorant:

• XPath parsing
• XML parsing

This means that false positives may be encountered when elements differ only by namespace. Quoting https://tools.ietf.org/html/rfc8040#section-4.8.4:

The format of this parameter is an XPath 1.0 expression [XPath] and
 is evaluated in the following context:
o The set of namespace declarations is the set of prefix and
 namespace pairs for all supported YANG modules, where the prefix
 is the YANG module name and the namespace is as defined by the
 "namespace" statement in the YANG module.

But the XPath construction in AbstractQueryParams does not set a proper NamespaceContext. Fix this by supplying a proper NamespaceContext backed by the EffectiveModelContext. Once this is corrected, use UntrustedXML to parse the XML to prevent any shenanigans.