Index SchemaNode substatements lazily

There is a slight impedance between SchemaNode and EffectiveStatement interfaces, where SchemaNode provides a view of a certain class of substatements – for example UnknownSchemaNode, NotificationDefinition, etc.

Since our statement implementations have all statements and we do not always use these views, we can make these derived collections lazily-computed with the use of VarHandles (to eliminate volatile reads).