messages that may have been received in the init phase are flushed immediately…

messages that may have been received in the init phase are flushed immediately after engine is plugged in

src/zmq_engine.cpp

@@ -58,6 +58,9 @@ void zmq::zmq_engine_t::plug (i_inout *inout_)
     set_pollout (handle);
 
     inout = inout_;
+
+    //  Flush all the data that may have been already received downstream.
+    in_event ();
 }
 
 void zmq::zmq_engine_t::unplug ()