The frontends use the new core actor, while the core actor uses the backend.
This is a step towards supporting multiple backends, where the core actor will
coordinate the backends.
- Rename .frontend.MpdFrontend to .dispatcher.MpdDispatcher, as it
dispatches requests.
- Move exceptions into .exceptions.
- Move .server.MpdSession to .session.MpdSession.
- Move handle_pattern to .protocol.handle_pattern.