Refactor invalidate and exipre

ObjectLock.invalidate() is called if anything goes wrong during a transaction to indicate that the entity should not be put back into cache as it may have an invalid state (at least as far as I understand this). To do so ObjectLock.invalidate() internally sets _invalidated flag. This flag is checked at ObjectLock.release() to set _object to null. But in contrast to my expectations the flag isn't checked at TypeInfo.release() to determine if the object should be put back into cache or not. Instead a CacheEntry with Object[] being null is put into cache.

On the other hand we have an mechanism to expire an entity. From my point of view the expiration is called by a user to tell Castor that the entity in cache is invalid which is about similar to the situation where Castor detects an invalid entity state itself.