Graduate ChildSchemaNodeNamespace as SchemaTreeNamespace

This is a @Beta construct in yang-parser-rfc7950, but it provides
a crucial link between statements and namespaces.

In terms of semantic change, this is a direct replacement for the SPI
idea that there is a SchemaNodeIdentifierNamespace. While that namespace
allowed lookup based on SchemaNodeIdentifier, while
SchemaTreeNamespace follows QName addressing -- i.e. we are changing
the addressing mode and allowing more flexible walks.

This is a strictly speaking API-breaking change, but since it enables
reactor/parser-rfc7950 interactions and is allowed under @Beta API
change rules, we will backport this to otherwise stable branches.

Existence of this namespace allows us to neatly tie it with
OnDemandSchemaTreeStorageNode.

JIRA: YANGTOOLS-1168
Change-Id: I9b1c30d21f7021d4c21f3e5a519f6cd1539871ad
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit 6682a8e2a8c02ed7a66bddb8b7a86009ffbbb98d)