Avoid plucking Todo ids in TodoService

What does this MR do?

TodoService should not call .select(&:id) on todos, because this is bad performance. So instead use sub-queries, which will result in a single SQL query to the database.

https://docs.gitlab.com/ee/development/sql.html#plucking-ids

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

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#30374