Support converting multiple columns in a single migration

As part of the effort to convert primary keys to bigint, we discovered some tables have more than one column that needs to be migrated. For example, ci_job_artifacts has both the id column and the job_id column (foreign key referencing ci_builds) which need to be converted to bigint.

We need to support this, to prevent having to make more than one pass over the table. This change includes updating both the trigger and background migration code to support setting multiple columns.