Give the Rails cache its own Redis namespace

Before this change, Rails cache data was stored in a global Redis
namespace. As a consequence, clearing the Rails cache (`rake
cache:clear`) would also delete all Sidekiq queue data and session
storage. This change puts all Rails cache data in a `cache:gitlab`
namespace, making `rake cache:clear` safe again.

CHANGELOG

@@ -17,6 +17,7 @@ v 6.8.0
   - Fix download link for huge MR diffs
   - Expose event and mergerequest timestamps in API
   - Fix emails on push service when only one commit is pushed
+  - Store Rails cache data in the Redis `cache:gitlab` namespace
 
 v 6.7.3
   - Fix the merge notification email not being sent (Pierre de La Morinerie)

config/environments/production.rb

@@ -53,7 +53,7 @@ Gitlab::Application.configure do
                else
                  "redis://localhost:6379"
                end
-  config.cache_store = :redis_store, resque_url
+  config.cache_store = :redis_store, resque_url, {namespace: 'cache:gitlab'}
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server
   # config.action_controller.asset_host = "http://assets.example.com"

config/initializers/session_store.rb

@@ -2,7 +2,7 @@
 
 Gitlab::Application.config.session_store(
   :redis_store, # Using the cookie_store would enable session replay attacks.
-  servers: Gitlab::Application.config.cache_store.last, # re-use the Redis config from the Rails cache store
+  servers: Gitlab::Application.config.cache_store[1], # re-use the Redis config from the Rails cache store
   key: '_gitlab_session',
   secure: Gitlab.config.gitlab.https,
   httponly: true,