Use Rails 5.2 Redis caching store
In GitLab v12.2.0, we enabled the Rails Redis caching store in production (https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30966). However, we had to to revert this in 6bda359b because Rack Attack v4.4.1 wasn't fully compatible with this store and caused fatal errors when rate limits were enabled (https://github.com/kickstarter/rack-attack/issues/281).
Now that we have upgraded to Rack Attack v6.2.0, which pulls in https://github.com/kickstarter/rack-attack/pull/340, this caching store is now supported. This is validated by the GitLab QA tests introduced in gitlab-foss!32177 (merged).
Edited by Stan Hu