Add OnDemandSchemaTreeStorageNode

Add OnDemandSchemaTreeStorageNode

In order to correctly implement schema dependencies, we need to
invoke a callback from namespace prerequisites back to
StatementContextBase and its subclasses.

The conduit here is NamespaceBehavior.getFrom(), which is invoked
with a namespace key. For ChildSchemaNodeNamespace this ends up
the QName matching the argument the corresponding
SchemaTreeEffectiveStatement will end up having.

Introduce OnDemandSchemaTreeStorageNode, which acts as an extension
to normal NamespaceStorageNode behavior -- it allows an
implementation to supply a QName-based child statement to be
returned.

Since StatementContextBase is a NamespaceStorageNode, its subclasses
can now additionally implement this interface to receive the
callback.

JIRA: YANGTOOLS-784
Change-Id: I0e6f621eb39b341af61005b2334174e814c18875
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>