MEDIUM: stream: move the listener's pointer to the session

The listener is session-specific, move it there.
diff --git a/include/types/session.h b/include/types/session.h
index c07b46a..7ee7fb2 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -36,6 +36,7 @@
 #include <types/stick_table.h>
 
 struct session {
+	struct listener *listener;      /* the listener by which the request arrived */
 };
 
 #endif /* _TYPES_SESSION_H */