This new delegate method will be called generally once per app start, to always provide a current token. This token may change over time. This simpler method makes integration much simpler, as:
* Developers no longer have to check for a current token using the `.fcmToken` property, and also check for token changes using the `-messaging:didRefreshRegistrationToken:` delegate method.
* There is a single code path for when a token is available, making operations that depend on a token being available easier to implement. For example, this is the right method to always upload your FCM token to your application server, or to subscribe to topics, etc.
Paul Beusterien