Reconstruct inference stack during normalization

Reconstruct inference stack during normalization

Centralize allocation of Inference, adjusting tests to take advantage of
this facility. This also hides the public InstanceIdentifierContext, so
that we get clarity as to the actual context in which we are executing.

AbstractFieldsTranslatorTest is refactored to remove mocking and
correctly bind to the undelying model.

This temporarily picks up yang-model-util-7.0.15 for
restconf-{common,nb-rfc8040}, so as to gain access to new methods of
DataSchemaContext{Node,Tree}.

JIRA: NETCONF-818
Change-Id: Iaf3362877261f3caaebfb4ec98ef6de792139f12
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>