Improve backstage performance of CI

This issue is to track meta improvements for ~CI in ~backstage that improves overall performance.

Related

• Optimise build picking: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9511,
• Persist stages in database: https://gitlab.com/gitlab-org/gitlab-ce/issues/26481,
• Show persisted stages in the UI instead of calculating that on the fly.
• Store information about retry in database: https://gitlab.com/gitlab-org/gitlab-ce/issues/25737,
• Deprecate old runners: https://gitlab.com/gitlab-com/infrastructure/issues/1303,
• Optimise pipelines json: https://gitlab.com/gitlab-org/gitlab-ce/issues/27377,
• Update pipeline status synchronously: https://gitlab.com/gitlab-org/gitlab-ce/issues/23257,
• Improve support for handling big pipelines,

(additional issues will be added once we figure out them)