We cannot clear the custom variable tables on startup and then re-insert
all known custom vars. Instead we "fixed" it by leaving them
in the database only updating their value if changed. We certainly
forgot about deleted custom vars at that point.
Since the older fix must stay in place, we'll use a session
token stored in the backend to check whether the custom
variable has been updated/inserted in the current session, or not.
If the session token does not match, the custom vars are deleted
at the end of the config dump. Ugly but fairly the only performant
solution as CVs don't have an object_id.
fixes#10436