Use a ConnectionPool for Rails.cache on Sidekiq servers

It’s important to remember that connections on ConnectionPool are created 
when needed so if modify the objects later to create those connections weird 
things could happen

https://gitlab.com/gitlab-com/infrastructure/issues/464#note_15850653

CHANGELOG

@@ -6,6 +6,7 @@ v 8.13.0 (unreleased)
   - Log LDAP lookup errors and don't swallow unrelated exceptions. !6103 (Markus Koller)
   - Add more tests for calendar contribution (ClemMakesApps)
   - Fix robots.txt disallowing access to groups starting with "s" (Matt Harrison)
+  - Use a ConnectionPool for Rails.cache on Sidekiq servers
   - Only update issuable labels if they have been changed
   - Revoke button in Applications Settings underlines on hover.
   - Update ruby-prof to 0.16.2. !6026 (Elan Ruusamäe)

config/application.rb

@@ -116,6 +116,10 @@ module Gitlab
     redis_config_hash = Gitlab::Redis.params
     redis_config_hash[:namespace] = Gitlab::Redis::CACHE_NAMESPACE
     redis_config_hash[:expires_in] = 2.weeks # Cache should not grow forever
+    if Sidekiq.server? # threaded context
+      redis_config_hash[:pool_size] = Sidekiq.options[:concurrency] + 5
+      redis_config_hash[:pool_timeout] = 1
+    end
     config.cache_store = :redis_store, redis_config_hash
 
     config.active_record.raise_in_transactional_callbacks = true