[Broadcaster] Distribution issues

[sgfn]

Umbrella for various issues present ATM when running Broadcasters in a cluster:

• [ ] stream name and description are per-instance
• [ ] chat history is per-instance
• [ ] chat nickname registration is per-instance
  • possible fix: cluster-wide registry for chat nicknames, like the one used for PCs
• [ ] deleting PC fails when request is issued to the wrong instance
  • possible fix: multicall to all supervisors in PeerSupervisor.terminate_pc/1
• [ ] remote inputs from disconnected/dead nodes remain dangling