Invalidate namespace descendants cache

What does this MR do and why?

This MR implements invalidation of namespace hierarchy cache when change happens in the hierarchy:

• parent_id changes
• namespace is deleted

In this MR we implement the invalidation with ActiveRecord callbacks to minimize the impact on the system. With a feature flag we can gradually roll it out and see if there is any negative impact. At some point later the invalidation might be implemented as a SQL trigger.

Invalidation logic

When namespace is moved (parent_id changes) or new namespace is created:

• Take the current parent id and the previous parent id and invoke the expire_for method.
• The expire_for method invokes a SQL query where all parent ids until the top level namespace (basically traversal_ids) are collected and then expired by bumping the outdated_at column of the cached records (if present).

When namespace is deleted:

• Take the current id and parent namespace ids and invoke the expire_for method.

Related epic: &11469

Worst case the expire_for method would access around 40 rows (assuming maximum hierarchy depth of 20), in reality this will be only a few rows.

Database

• Query when no cache data is present: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/25189/commands/80019
• Query when some cache data is present: https://console.postgres.ai/gitlab/gitlab-production-tunnel-pg12/sessions/25189/commands/80047

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Related to #433482 (closed)