3.0 Convert Cache + Behavior classes to use _config

As outlined in https://github.com/cakephp/cakephp/issues/2298, I've converted the behavior + cache classes to use `_config` instead of `settings`. I've left long standing public methods alone, as changing the public interface will cause further compatibility breaks.

I didn't touch the Component or Helper classes as I think additional public methods will be required, and for example making PaginatorComponent's settings protected will break Controller::paginate().