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/router.cpp

@@ -56,6 +56,7 @@ zmq::router_t::router_t (class ctx_t *parent_, uint32_t tid_, int sid_) :
     options.recv_routing_id = true;
     options.raw_socket = false;
     options.can_send_hello_msg = true;
+    options.can_recv_disconnect_msg = true;
 
     _prefetched_id.init ();
     _prefetched_msg.init ();