Add `db_spin` and refactor ChaosController

What does this MR do?

Adds a bunch of improvements:

• Removes GITLAB_ENABLE_CHAOS_ENDPOINTS=1 as this seems not needed for dev environment, where it should be always enabled,
• Requires the usage of GITLAB_CHAOS_SECRET for non-development environments making this the only one option to configure,
• Tidy the ChaosController code,
• Add db_spin that interleaves CPU intensive with DB access which seems to be the most common usage pattern at GitLab,

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/64425

Does this MR meet the acceptance criteria?

Conformity

• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides