problem: router doesn't know when peer disconnected

ZMQ_ROUTER_NOTIFY doesn't have a context and doesn't play nice with protocols. with ZMQ_DISCONNECT_MSG we can set it to a protocol message, like DISCONNECT in majordomo. Router will send it when a peer is disconnected. Another advantage of ZMQ_DISCONNECT_MSG is that it also works on inproc.

Together with ZMQ_HEARTBEAT it allows to build very reliable protocols, and much simpler as well.

src/options.hpp

@@ -301,6 +301,10 @@ struct options_t
     //  Hello msg
     std::vector<unsigned char> hello_msg;
     bool can_send_hello_msg;
+
+    //  Disconnect msg
+    std::vector<unsigned char> disconnect_msg;
+    bool can_recv_disconnect_msg;
 };
 
 inline bool get_effective_conflate_option (const options_t &options)