Checkpoint timers as part of managed keyed state instead of raw keyed state

Timer state should now become part of the keyed state backend snapshot, i.e., stored inside the managed keyed state. This means that we have to connect our preparation for asynchronous checkpoints with the backend, so that the timers are written as part of the state for each key-group. This means that we will also free up the raw keyed state an might expose it to user functions in the future.