[EE] Add a memory cache local to the thread to reduce Redis load

Loading ApplicationSetting from Redis was responsible for at least 50% of the CPU load of the Redis cluster on GitLab.com. On GitLab.com, our serialized ApplicationSetting is a whopping 54K due to the large number of domain blacklists. Enabling compression for Redis.cache would drop this down to 20K, but that is still large.

Since these settings generally don't change very much, we can load this from the database and cache it in memory, skipping Redis altogether. We use ActiveSupport::Cache::MemoryStore as a drop-in replacement for RedisCacheStore even though we probably don't need synchronized access within Thread.current.

The main downside with this approach is cache invalidation: all Unicorn and Sidekiq processes cache their values for a minute before refreshing from the database. For routes under /admin/application_settings, we adjust the controller to load the uncached values to prevent stale data from being inserted.

For the usage ping consent case, we also load an uncached value and tighten up the conditions when the query is run.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63977

Backport of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30233/diffs.