Please implement an API like func (srv *Server) Serve(c net.PacketConn) error

Please add an new api in server.go
The code like below:
```
func (srv *Server) Serve(c net.PacketConn) error {
	srv.addConn(c)
	defer srv.removeConn(c)
	return srv.agent.ServePacket(c)
}

```

I want to add another net.PacketConn implement not using net.ListenPacket, and I cann't find any api to support.

Thanks for you great project!