Support zero-downtime upgrades for Rails application
Currently upgrading the Rails application or simple config changes require a restart with downtime. We should try to find a way to remove this downtime from the procedure.
As GitLab Rails running in Puma can't do this internally, we need to do this externally by running multiple Raiis applications (and workhorses) behind a load-balancer.
This requires a whole bunch of other changes: