[Feature Request](s) reflect db-level uniqueness violations in errors, use DB time for created_at/updated_at

If you want reliable uniqueness on a field, a DB-level constraint is necessary. But DB-level constraints are not reflected in #errors like other validations. We implemented this in an around_filter at my work, would core be interested in a PR? I’m pretty open to whatever API people want-the most unobtrusive one I can think of would be to add an option to validates_uniqueness_of, perhaps :enforced_in_db or something like that.

Unrelated: server clocks tend to drift, so often the created_at timestamps of a set of AR objects sort in a different order than their ids. It wouldn’t be terribly hard to use select now() instead of Time.now for those timestamps. Any interest in a PR for that?

The now() option would be nice, but keep in mind that the syntax for that is DB-specific, sadly.