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
Also here: are there cases not handled when using `$targetClassMetadata->identifier[0]`? The worrying part is just the hardcoded `0`