ConvergentGossip: automatic discovering of peers

[ktoso]

The gossip should be able to get a function from Member -> ActorRef<Message>? so when a node joins or is made up, the gossip may start to include it in it's gossip rounds or not.

Currently we manually provide it all refs it needs since we use it "very early in the cluster".

This should be such that it can work either:

• with just the receptionist, so I guess we simply provide it a ReceptionKey and it subscribes for it
• manual Member -> Ref could be useful where we do not want to use receptionist for some reason? Need to rethink if there can be such cases, perhaps in the receptionist itself -- since there we can't rely on the receptionist to provide us the refs, since WE are that receptionist that needs to find other receptionists :)

[ktoso]

TODO in code

                case ._clusterEvent(let event):
                    fatalError("automatic peer location is not implemented") // FIXME: implement this https://github.com/apple/swift-distributed-actors/issues/371

[ktoso]

Convergent gossip is no more, and this IS implemented in Gossip<> 🎉