Gitlab::Redis bigger connection pool on sidekiq workers

What does this MR do?

This MR configure Gitlab::Redis to use a pool with a size related with the number of threads running the current process.

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

Are there any race condition setting the pool class instance variable?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• 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 you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Relates #18663 (moved)