Unique field validation with multiple DB connections.

[SardarNL]

django.db.models.base._perform_unique_checks is not using self._state.db to lookup unique fields. Check the following:

class MyModel(models.Model):
   my_unique = models.IntegerField(unique=True)

obj = MyModel(my_unique=value_exists_on_default_new_on_other_connection)
obj._state.db = other_connection
obj.full_clean()

# for existing objects _state.db will be already set
obj = MyModel.objects.using(other_connection).get(pk=existing)
obj.my_unique = value_exists_on_default_new_on_other_connection
obj.full_clean()

full_clean() will validate_unique() -> _perform_unique_checks(), which will eventually qs = model_class._default_manager.filter(**lookup_kwargs). The default manager is always using default connection, so unique check will be performed on default connection. Other validators, such as ForeignKey lookup, properly use .using() to keep talking to the database where the object came from.

Fix: qs = model_class._default_manager.filter(**lookup_kwargs).using(self._state.db)
The same fix is needed for _perform_date_checks().

[Tim Graham]

Duplicate of #15130