[DependencyInjection][3.0] Add initialized() to the ContainerInterface

The initialized() method has been added to the container in order to check whether a service is already initialized quite late during Symfony 2 development (#3557). In order to prevent a BC break a new interface was added (IntrospectableContainerInterface).

Let's clean that up in 3.0 and move initialized() to the ContainerInterface and drop IntrospectableContainerInterface.