EventSource: Require implementors to implement Cloneable

We already clone EventSource objects as seen in the previous commit.
SubscriptionManager.Subscription: Add
abstract storeNotificationWithoutCommit()

- The function is already present in all implementing child classes, so
its a good idea to add it here.
- Bonus changes while I'm at it anyway: The visibility is trimmed to the
lowest level, and the parameters are made final.
SubscriptionManager.IdentitiesSubscription: Move function upward

storeSynchronizationWithoutCommit() will be the same for all
implementations of Subscription, so this commit moves it to the base
class Subscription.