Uses AtomicReference to synchronize multiple thread access on Gitlab::Redis

Uses AtomicReference to synchronize multiple thread access on Gitlab::Redis. Concurrent-ruby is already a dependency of Gitlab (through Sprockets). These changes, adds it as a direct dependency for Gitlab.

For more information: https://gitlab.com/gitlab-org/gitlab-ce/issues/21598

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• Tests
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/21598

cc @jacobvosmaer-gitlab