Avoid sequential scans loading schema_migrations table when loading application settings

This was causing significant performance problems in production, and this commit
reverts to the original behavior.

lib/gitlab/current_settings.rb

@@ -24,7 +24,20 @@ module Gitlab
       private
 
       def ensure_application_settings!
+        cached_application_settings || uncached_application_settings
+      end
+
+      def cached_application_settings
         return in_memory_application_settings if ENV['IN_MEMORY_APPLICATION_SETTINGS'] == 'true'
+
+        begin
+          ::ApplicationSetting.cached
+        rescue ::Redis::BaseError, ::Errno::ENOENT, ::Errno::EADDRNOTAVAIL
+          # In case Redis isn't running or the Redis UNIX socket file is not available
+        end
+      end
+
+      def uncached_application_settings
         return fake_application_settings unless connect_to_db?
 
         current_settings = ::ApplicationSetting.current