Make InMemoryDataTreeSnapshot implement SchemaContextProvider
Note DataTreeSnapshot's duplication of getSchemaContext(), which
is indicative of a coupling to yang.model.api. This is fine for now,
as this coupling already exists through xpath interfaces, but it
needs to be revisited and cleaned by either removing the method,
or making it a non-default (and inherited from SchemaContextProvider).
Change-Id: If335f178e9e086b8b5185044a06d4296f81b2a74
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>