Preserve constraint names

Constraint names are now preserved, index, join and foreign keys will store it. It will contain a generated or a given name. Constraint names must now be unique within a schema. Foreign key naming is now done by the DefaultNameGenerator, ensuring unique name generation. Join names are also generated only by the DefaultNameGenerator.

Constraint names can now be used as a handle to drop the constraint.

 

This isn't ever used, right? If so, an IllegalArgumentException might be better.