Revert "Don't send welcome message to subscribers. Wait for an application event instead"

This reverts commit 19fd1bde79fbbecc7dc6e489d748f04724fd3c0d.
Tracking application events seems like a more robust solution but also more complex.
Since the broker is running in another process, we would need to add another socket connection and listen on this socket for the events.
Split setting up servers and start listening for connections
Stop using hardcoded event ids, generate randome ones
Implement stop receiving updates after unfollowing