Survey/Report sockets

[Karrq]

msg-rs currently supports 2 communication patterns:

• req/rep, 1-1
• pub/sub, 1-N with no replies

It could be useful to support an additional 1 to N communication, whilst also allowing peers to reply and streaming those replies to the application code

[mempirate]

In zmq, this same behaviour is achieved by letting request sockets connect to multiple reply sockets. We could do this also, but it would roughly turn the socket.request() -> Reply interface into socket.request() -> Stream<(Reply, Source)>.