Geo: Decrease frequency of project shard schedulers when few projects to schedule

What does this MR do?

Adds a backoff time to Geo::RepositoryShardSyncWorker when there were no pending resources to sync.

The Geo::RepositoryShardSyncWorker is a backfilling job.

It's useful when the instance is bootstrapped and still has no data. If the instance is healthy and no data was manually removed from a secondary node, then the backfill would not have anything to work at anyway. It's still an important part of Geo as it works as a safe layer for when the normal synchronization fails.

What are the relevant issue numbers?

#5748 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?

Closes #5748 (closed)