MINOR: session: introduce session_new()

This one creates a new session and does the minimum initialization.
diff --git a/include/proto/session.h b/include/proto/session.h
index 3f008e1..cb4deec 100644
--- a/include/proto/session.h
+++ b/include/proto/session.h
@@ -33,6 +33,7 @@
 #include <proto/stick_table.h>
 
 extern struct pool_head *pool2_session;
+struct session *session_new(struct proxy *fe, struct listener *li, enum obj_type *origin);
 void session_free(struct session *sess);
 int init_session();
 int session_accept_fd(struct listener *l, int cfd, struct sockaddr_storage *addr);