FCPInterface: Implement sending of BeginSynchronizationNotification

- Will be used by
SubscriptionManager.*Subscription.notifySubscriberByFCP()
- Insipired by FCPInterface.handleSubscribe() / sendChangeNotification()
SubscriptionManager.IdentitiesSubscription: Use
FCPInterface.sendBeginOrEndSynchronizationNotification()
FCPInterface: Remove sendAllIdentities(), not used anymore

- Was being used in
SubscriptionManager.Subscription.synchronizeSubscriberByFCP(), which has
been replaced by the recent commits which send synchronizations as
regular ObjectChangedNotifications
- Also, remove support for calling handleGetIdentities() with
request=null since sendAlllIdentities() was the only user of that
feature.