OGM-299 Use SessionFactoryServiceInitiator instead of sessionFactory.addObserver to inject SessionFactory in DatastoreProvider
OGM-299 Clean DatastoreProviderInitiator JavaDoc
OGM-299 Remove unnecessary parameter in DatastoreProviderInitiator.create
OGM-299 Add JavaDoc on SessionFactoryRegistry services

The reason why some services are SessionFactoryRegistry based is not
obvious. Adding JavaDoc to help readability.