Optimise pipelines.json

What does this MR do?

Following and looking at this MR this is another take on that: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10327.

This effectively (currently the biggest problem is stages) reduces number of queries from ~200 to ~50 on my test setup. It reduces the request processing time by 4-5x.

This is a proof of concept.

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

Why was this MR needed?

Screenshots (if relevant)

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?