Cluster workers refactoring

What does this MR do?

This is one of ~"technical debt" issues. See https://gitlab.com/gitlab-org/gitlab-ce/issues/39837.

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Context:

• https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/14879/diffs#note_45811464

I wish we could use Clusters module in app/workers/ too, do you think it would be possible?

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

Why was this MR needed?

Screenshots (if relevant)

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

What are the relevant issue numbers?

Related #39837 (moved), !14879 (merged)