rollback functionality added to pipe

src/pipe.hpp

@@ -100,6 +100,9 @@ namespace zmq
         //  message cannot be written because high watermark was reached.
         bool write (zmq_msg_t *msg_);
 
+        //  Remove any unflushed messages from the pipe.
+        void rollback ();
+
         //  Flush the messages downsteam.
         void flush ();