How do you add an incrementing primary key to an existing table?
This actually seems to be an issue in Drupal Core where creating a serial field on an existing table is not possible in MySQL 5.7+ without first creating an int field and converting that to autoincrement (this issue does not show up in MySQL 5.6 and below). Please see https://www.drupal.org/node/2615496 for a patch that fixes this issue for new releases of MySQL. It might be worthwhile to update that patch to be rolled into 8.x so it will allow us to patch 7.x.