Refactor riak_api_pb_service to use callback mod attributes

Use `-callback` module attributes instead of `behavior_info` to declare the callbacks. This plays better with dialyzer and will get rid of all the `Callback info about the riak_api_pb_service behaviour is not available` warnings across the projects.

Also updated the `decode` callback attribute and edoc to include the optional security namespace.

@Vagabond @seancribbs Might want to give a quick look around my "security namespace" addition. I'm not 100% that is the best name or if I documented it properly.