ModelPersistence: for the time being, PersistenceVersionAware is implemented through DefaultSModel+SModelHeader, to support existing code. Future approach shall (a) use openapi.SModel, not SModelData in persistence implementation (presently limiting us to access to all properties through SModelData) (b) get rid of direct smodel.SModel use and rely on SModelData interfaces
ConceptDescriptor: towards slim descriptors - replace uses of ConceptDescriptor.getAncestors with SConcept alternative
ConceptDescriptor: drop unused methods in LanguageHierarchyCache; javadoc.
ModelPersistence: drop FilePerRootSModel, replace with DefaultSModelDescriptor (implementation is identical) - no reason for openapi.SModel implementation to care about specific way it's going to be serialized.