use reactive-rabbit and newest akka-streams

Quite a lot changes to the API.

For update:
- [x] code
- [ ] comments
- [ ] tutorial
What do you think about chagning result type of `consumerMapping` to `Flow[Delivery, String]`? Method would like this:

```
def consumerMapping: Flow[Delivery, String] =
  Flow[Delivery].map(_.message.body.utf8String)
```

You don't need `ByteString` for anything so you can get rid of that before entering `domainProcessing`.