Prevent Sidekiq sleep when DB replica is up-to-date

What does this MR do and why?

This is a follow-up to !74532 (merged)

Based on previous tests, even without any delay, 50% of jobs get picked up when the replica is already up-to-date. This change skips the sleep for those cases.

In the worst case, this adds an extra WAL check query, but these are very fast queries.

Related to gitlab-com/gl-infra/scalability#1380 (comment 756167450)

This is behind the skip_scheduling_workers_for_replicas feature flag.

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