Meanwhile the above (which is the Observer for the mapObservable onto the cold
observable) will trigger everything always for all observers:
- `onSourceCompletion` triggers the complete callback on all observers of
`target`, and any observable like map-observable based on it will be triggered
to `complete` their observers as well
- `onSourceError` already makes it visible that it forwards errors from source
to all observers on the map-observable
- `mapSubscription` uses the `forward` proc which forwards the value to all
observers on the map-observable, not just a specific one