Deprecate CopyableNode at al.

Deprecate CopyableNode at al.

isAddedByUses(), isAugmenting() and similar constructs relate to how
a particular node became effective. This really has not place in the
effective model of the world.

This patch concentrates duplicate methods into an interface hierarchy
and marks them as @deprecated, along with justification. This allows
us to track down users and the semantics they really are after.

Since we are touching SchemaContext, mark its future direction, which
should see it shed a lot of nonsensical methods.

Change-Id: Ibdaf52e45076006e979204ac75e7bae6aeffd29a
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>