LIBZMQ-497 there could be unsent bytes in encoder

When we send a large message, the message can be splitted into two chunks.
One is in the encoder buffer and the other is the zero-copy pointer.
The session could get the term before the last chunk is sent.

src/stream_engine.hpp

@@ -135,6 +135,7 @@ namespace zmq
         std::string endpoint;
 
         bool plugged;
+        bool terminating;
 
         // Socket
         zmq::socket_base_t *socket;