Add helper for renaming columns using a background migration
We have a helper for changing column types using a background migration: https://docs.gitlab.com/ce/development/what_requires_downtime.html#changing-column-types-for-large-tables
We should:
- Add a similar helper for renames (the existing helper would almost work unchanged, as the background migration is generic enough to support it).
- Document that helper on the same page.
- Update https://gitlab.com/gitlab-org/gitlab-ce/blob/master/rubocop/cop/migration/update_large_table.rb to also detect
rename_column_concurrently
andchange_column_type_concurrently
, to remove any element of human error.