Refines are taking immediate effect and do not exist directly in
SchemaNode view of the world. Do not implement SchemaNode, which makes
things a tad easier.
JIRA: YANGTOOLS-1279
Change-Id: If8b30e6bbf87ed0de79d07e620f694de552460ce Signed-off-by: Robert Varga <robert.varga@pantheon.tech>