Improve rename projects migration

What does this MR do?

Makes migration single threaded

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

no

Why was this MR needed?

To simplify migration code and fix possible race condition

Does this MR meet the acceptance criteria?

• 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

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/29556 based on https://gitlab.com/gitlab-org/gitlab-ce/issues/29556#note_25512861