Initial proposal for DeltaInstanceInfo

I do not understand ```forDeltas``` method's intent. We should create a distinct notification type, and send just delta with it, like:
```java
public Observable<ChangeNotification<InstanceInfo>> forDeltas(final InstanceInfo baseInstanceInfo) {
        return Observable.from(deltas)
                .map(new Func1<Delta, ChangeNotification<Delta>>() {...});
}
```

On client side the flow would be:
* client sets own InstanceInfo
* in the channel we calculate the delta of type DeltaInstanceInfo
* we send individual field updates to the server side

On server side:
* We receive field level notifications
* Channel keeps latest full InstanceInfo object
* Channel creates a new instanceInfo by applying a delta
* Channel updates the registry with new, complete InstanceInfo object
The downside on server side is that for each delta we will do the merge operations, so if client updates N fields, that will trigger N protocol messages, and N merge/registry update operations. We could think about batch updates, or sending a ```final``` flag with each update.