[Request] custom load balancer/routers

[thijsterlouw]

Would it be possible to make the load-balancer for the 2.1 branch of ZeroMQ more easily customizable?

I see mentions to lb_t inside push and xreq but the lb_t class doesn't appear to be extensible because the lack of virtual functions? It would be nice if I could plug in my own custom load balancer based on for example weights, latency (coupled with more details about the internal status of sockets possibly) etc.