Prevent parallel backend instances during database migrations

[chaoran-chen]

Originally brought up in #3125:

When we deploy a new version, Kubernetes will usually do a rolling update where it first starts the new backend and then shuts down the old one, right? If yes, this means that if we do a database migration, the old backend could write values into the database that are inconsistent with the new database version and thereby put the database into an invalid state. Can we somehow avoid having two parallel running backends (at least) if there is a database migration?