Include ZMTP properties in message metadata

Metadata are built in stream_engine now.
This makes it easy to extend metadata with user-defined properties.

src/stream_engine.hpp

@@ -30,6 +30,7 @@
 #include "options.hpp"
 #include "socket_base.hpp"
 #include "../include/zmq.h"
+#include "metadata.hpp"
 
 namespace zmq
 {
@@ -131,6 +132,9 @@ namespace zmq
         size_t outsize;
         i_encoder *encoder;
 
+        //  Metadata to be attached to received messages. May be NULL.
+        metadata_t *metadata;
+
         //  When true, we are still trying to determine whether
         //  the peer is using versioned protocol, and if so, which
         //  version.  When false, normal message flow has started.