Seeing all the changes replayed when joining a realtime session is not nice

When joining an existing realtime session, you can see for a few seconds the changes from the realtime session re-played, which is not nice (lots of flickering text). Would be better to hide this under a loading animation. Moreover, we need to double-check that the checkpoint feature of Netflux / Chainpad is really working, i.e. make sure that we're not replaying all the changes, since the start of the realtime session, but only the change since the last checkpoint.