Fixed a bug so that a versioned entity with a oneToOne id can be created

- Fixed the basic entity persister so that versioned OneToOne entities can be created
- Created an IdentifierConverter utility class
- Moved the logic for the flatten identifier method into the new utility class
- Replaced calls for private flattenIdentifier to use new utility
- Added appropriate unit tests
I think the `Flattener` is better here. Not a nice naming either, but `Converter` doesn't say much, and seems to assume a bidirectional conversion.