Make sure database migrations are truly backwards compatible

This is one of several issues that was raised in https://gitlab.com/gitlab-com/infrastructure/issues/1944#note_38240450 and is required to unblock https://gitlab.com/gitlab-org/gitlab-ce/issues/34877

@stanhu has noted: "This should be the case with the zero-downtime deploys, but it's possible we hit edge cases where something goes wrong."

@yorickpeterse @_stark what tools are available in the toolbox to help accomplish this?