Missing notifications for calls and messages on a federated homeserver

The first report was about missing call notifications but notifications for messages can miss too.

Riot iOS rageshakes:

1. https://github.com/matrix-org/riot-ios-rageshakes/issues/1606
The log only shows a 20s latency on the notification reception with a timeline like:
```
20:34:24 You open riot-ios
20:34:26 You answered the call
20:34:28 The app gets the notification
20:39:05 You ended the call
```
2. https://github.com/matrix-org/riot-ios-rageshakes/issues/1622
To analyse