This moves the .service property from Server to Peer, as it's technically applicable to either.

Additionally, this adjusts how to use a custom class for the config (defaults to Config) and the service (defaults to Interface). Instead of passing a custom class when calling peer.config, the custom class must be specified at the instantiation of the Peer (or later on via peer.config_cls) and peer.config is no longer callable.

The same customization option is available for peer.service_cls and peer.service remains uncallable, as it was when it was only available on Server.

The .config change is a breaking change, but was deemed to be a cleaner overall solution, and is now consistent with how .service was being referenced.