[IMP] improve overridden sql constraints support

If module A define an sql constraints and module B, which depends on 
module A, override the sql constraint, we must ensure having only the 
last definition in constraints list. Having the two versions in the list
makes the ORM continuously trying to drop the second version by the 
first one, then recreate the second version, when performing operations 
like an upgrade a module B.

An illustration of this is the login_key on res.users, which is unique
per login in base, but unique per (login, website) in website.

closes odoo/odoo#30334