Redis caching backend should be available and enabled also in the master instance

While default global.inc file always included the code required to integrate caching backend, it was never really used, which is silly, despite the fact that we discourage people from using master instance, hence it is a bug and not a feature request. We should manage automatically added Redis caching backend also to all sites hosted on the master instance out of the box, which is easy, because new backend doesn't require any extra module enabled, just present in the expected directory.