Started init/start/uninit process refactor to add a stop() method to be able to destroy a Core in a more deterministic way than in the destructor.
Because on some garbage collected systems like Java/Android we can't rely on the unref/destructor method to correctly destroy the LinphoneCore when the app context is realeased by the system, we added a stop method that does the same. This way we can control when the ressources are freed. And this allows us to add back the notify global state when the core goes in shutdown/off state and also to start it back which simplifies core restart (until now it was a core destroy followed by the creation of a new Core).