MEDIUM: session: attach incoming connection to target on embryonic sessions

In order to reduce the dependency over stream-interfaces, we now
attach the incoming connection to the embryonic session's target
instead of the stream-interface's connection. This means we won't
need to initialize stream interfaces anymore after we implement
dynamic connection allocation. The session's target is reset to
NULL after the session has been converted to a complete session.
diff --git a/include/types/session.h b/include/types/session.h
index ba12636..a94ca1d 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -122,7 +122,7 @@
  */
 struct session {
 	int flags;				/* some flags describing the session */
-	enum obj_type *target;			/* target to use for this session */
+	enum obj_type *target;			/* target to use for this session ; for mini-sess: incoming connection */
 
 	struct channel *req;			/* request buffer */
 	struct channel *rep;			/* response buffer */