Convert primary to a secondary with respect to already synced data

This means that Geo will keep the event log around until the old primary is brought back as a secondary, allowing its state to be fast-forwarded from that time. Effectively, we'd be creating a pointer for the state of the old Geo primary at the time it goes offline.

See details https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4425#note_58435216