MINOR: sessions: Start to store the outgoing connection in sessions.

Introduce a new field in session, "srv_conn", and a linked list of sessions
in the connection. It will be used later when we'll switch connections
from being managed by the stream, to being managed by the session.
diff --git a/include/types/session.h b/include/types/session.h
index c3bace8..e0e1455 100644
--- a/include/types/session.h
+++ b/include/types/session.h
@@ -47,6 +47,8 @@
 	struct vars vars;               /* list of variables for the session scope. */
 	struct task *task;              /* handshake timeout processing */
 	long t_handshake;               /* handshake duration, -1 = not completed */
+	struct connection *srv_conn;    /* Server connection we last used */
+	struct list conn_list;          /* List element for the session list in each connection */
 };
 
 #endif /* _TYPES_SESSION_H */