HHH-8990 Improve backwards compatibility of 4.2 branch by reintroducing ...

...the deleted NonFlushedChanges interface

https://hibernate.atlassian.net/browse/HHH-8990

@brmeyer turns out this is all I really need to get Hibernate Search to work on the full 4.2 range: we can ignore some of those methods as they are just compile-time dependencies, so the only remaining problem is that to compile with the older version it would import this interface.

@sebersole a good compromise?