Do not run XML encoding before RESTCONF filter

Current implementation of query filters relies on XML serialization/deserialization before we determine if a piece of data actually matches the filter.

This is inefficient, as it involves a lot of dancing around, especially since we have XPath support available via yangtools' org.opendaylight.yangtools.yang.data.api.schema.xpath package, which is implemented by yang-data-jaxen and operates directly on SchemaContext/Normalized nodes.