Handle scheduled/retry sets in namespace migration

The schedule, retry, and dead sorted set needs to be migrated. As they are not time sensitive, we could run a migration promptly after the env-var toggle. The schedule sorted set is used for scheduled jobs (e.g. perform_at, perform_in) while retry is used to house failed jobs for retries.

Refer to thanos to check the GitLab SaaS's usage of the schedule and retry set.

One method would be to use ZUNIONSTORE to write data from the namespaced set into the non-namespaced one (&944 (comment 1325177170)).

The alternative is to patch the schedule set poller to poll both namespaces.