Refactor TypedSchemaNode

Refactor TypedSchemaNode

Factoration of WhenConditionAware is flushing out a mismatch between
Annotation and TypedSchemaNode.

TypedSchemaNode should really be called TypedDataSchemaNode, as it
captures nodes which are valid in datastore contexts and capture real
data.

Annotation, while sharing the aspect of being type-aware, is not normal
data as implied by DataSchemaNode. Notably it does not have a concept
of being added by augmentation nor being configuration.

In order to reconcile this mismatch, we really need a TypeAware trait,
which is mixed in with DataSchemaNode in TypedDataSchemaNode. Annotation
mixes it with UnknownSchemaNode -- resulting in more correct interfaces
being exposed.

Change-Id: Ie45b982c1a6ee3287ce57be34104b8f745a215ae
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>