Start with a clean Session in the unit-tests
- LinqExtension was already implemented in nHibernate v3.3.3.4001
- removed our implementation
small refactor ImmutableUserTypeBase
* DeepCopy returns value
* GetHashCode uses 0 if object is null

Let GenericWellKnownInstanceType inherit from ImmutableUserTypeBase
Remove Linq support
* IQueryOver is mature
* refactor friendly
* supports more features then Linq