Bad migration detection in multiple database setup

[Claude Paroz]

While trying to play with migrations in contrib apps, I stumble upon test issues, and finally found out that the migration was only applied to the first database ('default'), because while setting the second test database ('other'), MigrationRecorder was querying the first (default) database to detect if the migration was applied or not.

[Claude Paroz]

Use the proper database to query for Migrations

[Claude Paroz]

And just realized flush is a classmethod without access to self.connection :-/

[Claude Paroz]

Include classmethod fix and basic test

[Tim Graham]

Might be worth factoring self.Migration.objects.using(self.connection.alias) into a property on the class? Otherwise, looks good.

[Claude Paroz <claude@…>]

In 7c54f8cced37313572d72f59bd2ef7ce12aa8fe1:

Fixed #22474 -- Made migration recorder aware of multiple databases

Thanks Tim Graham for the review.

[Claude Paroz <claude@…>]

In 1084456ac2e707fc562906e95ad78f409dcc9325:

[1.7.x] Fixed #22474 -- Made migration recorder aware of multiple databases

Thanks Tim Graham for the review.
Backport of 7c54f8cce from master.