Add lazy flag to avoid circular reference

So that if you have the package "ocramius/proxy-manager", it will create proxy services for loaders. This avoids the circular reference when you use a callback which have a dependency on the factory (because the factory needs the loaders which needs the callback).

If you don't have the package installed, it doesn't change anything, cf. http://symfony.com/doc/current/components/dependency_injection/lazy_services.html