Geo - Avoid rescheduling the same project again in a backfill condition

What does this MR do?

Avoid rescheduling the same project again in a backfill condition. In a non-backfill condition, the Geo log cursor schedules a Geo::ProjectSyncWorker job when it gets a Geo::RepositoryUpdatedEvent event from the primary.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Somehow the scheduler is looping and rescheduling the same project again.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/5385