Refactor PostReceive worker to limit merge conflicts

What does this MR do?

Port changes made on https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1976 to CE.

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

No.

Why was this MR needed?

To limit conflicts with EE.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together