Device token for an iPhone device生成之后就永远不变吗？不是，if a device is wiped (应该是重装系统), it will get a new device token.
官方网站是这样写的： If the user restores backup data to a new device or computer, or reinstalls the operating system, the device token changes
正是因为device有可能改变，所以建议在app start时（即在didFinishLaunchingWithOptions 里）调用registerForRemoteNotificationTypes来获取device token以检查device token是否改变，如果改变了就应该把新token传给push provider。（官方描述：An application should register every time it launches and give its provider the current token）
device token应该存储在NSUserDefaults来达到新旧比较的目的
那么旧device token在push provider对应的record怎么办？
方案1：把旧device token send to provider and request delete record
方案2：使用apns feedback service。
方案2可能更好些，因为总是需要使用apns feedback service来处理用户在device里删除app的情况。
调用registerForRemoteNotificationTypes方法后，成功注册后，APNS就会返回一个device token，然后回调delegate methoddidRegisterForRemoteNotificationsWithDeviceToken, 如果注册失败，则回调delegate method didFailToRegisterForRemoteNotificationsWithError。
注意：
* 在第一次调用registerForRemoteNotificationTypes方法时没有联网，则既不会调用didRegisterForRemoteNotificationsWithDeviceToken，也不会调用didFailToRegisterForRemoteNotificationsWithError
*在第一次调用registerForRemoteNotificationTypes注册成功后，之后即使没有联网，再调用registerForRemoteNotificationTypes时都会以最上一次的device token作为参数回调didRegisterForRemoteNotificationsWithDeviceToken方法。
* (官方描述) If your application has previously registered, calling registerForRemoteNotificationTypes: results in the operating system passing the device token to the delegate immediately without incurring additional overhead.
from:
https://developer.apple.com/library/ios/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/IPhoneOSClientImp/IPhoneOSClientImp.html#//apple_ref/doc/uid/TP40008194-CH103-SW2
