Add BaseSchemaTreeStatementSupport statement policies
All current subclasses are using at least two broadly-applicable
policies:
- uninstantiated statements, i.e. action, notification, input, output
These are always EffectiveConfig.IGNORED anyway.
- instantiated statements, i.e. leaf, container and the like, who
need to adjust their view of EffectiveConfig.
Introduce these two and convert uninstantiated users.
JIRA: YANGTOOLS-1208
Change-Id: I7847bda9d9b5ef782deda4df40f7c449daafbf4f
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>