Only use RequestStore for current_application_settings during a request

This fixes an issue where the RequestStore was being populated with the settings in the unicorn master
during the rails initializers. Each forked worker would then start their first request with an uncleaned RequestStore.

lib/gitlab/current_settings.rb

 module Gitlab
   module CurrentSettings
     def current_application_settings
-      key = :current_application_settings
-
-      RequestStore.store[key] ||= begin
-        settings = nil
+      if RequestStore.active?
+        RequestStore.fetch(:current_application_settings) { ensure_application_settings! }
+      else
+        ensure_application_settings!
+      end
+    end
 
-        if connect_to_db?
-          settings = ::ApplicationSetting.current
-          settings ||= ::ApplicationSetting.create_from_defaults unless ActiveRecord::Migrator.needs_migration?
-        end
+    def ensure_application_settings!
+      settings = nil
 
-        settings || fake_application_settings
+      if connect_to_db?
+        settings = ::ApplicationSetting.current
+        settings ||= ::ApplicationSetting.create_from_defaults unless ActiveRecord::Migrator.needs_migration?
       end
+
+      settings || fake_application_settings
     end
 
     def fake_application_settings