Allow the creation of snaphots of non-Jdbc based databases

I'd like to create an extension for database diffs with JPA-annotated classes.
Currently most of the subclasses of `JdbcSnapshotGenerator` heavily depend on the database being an `AbstractJdbcDatabase`.

I want to create a Database-Class for the JPA-annotated classes which is not a subclass of `AbstractJdbcDatabase`. But since all subclasses have a priority > 0 they are considered as snapshot generators and then fail with exceptions.