Remove methods which expose internal ID

Remove the following methods with expose internal IDs
from public view

- DatastoreExtended#getDocNumericId() - Removed from public interface
- BasicDocumentRevision#getInternalNumericId() - Changed visibiity to protected

Only one question, should the getDocNumericId on BasicDatastore remain public or be made protected?
Actually I'm tempted to retract what I've just written. I can't really see threads sharing `DocumentRevisionTree` objects, as they're only every constructed on the fly by `getAllRevisionsOfDocument`