Keita Ichihashi

I have updated my comment with my analysis. I hope this helps with the review.

@dantownsend I think dropping a column has `db_column_name` from an existing table does not work as well. Perhaps need to set `db_column_name` instead of `column_name`. https://github.com/piccolo-orm/piccolo/blob/master/piccolo/apps/migrations/auto/migration_manager.py#L668