MINOR: lua: txn: create class TXN associated with the transaction.

This class of functions permit to access to all the functions
associated with the transaction like http header, HAProxy internal
fetches, etc ...

This patch puts the skeleton of this class. The class will be
enhanced later.
diff --git a/include/types/hlua.h b/include/types/hlua.h
index d05f59c..aada722 100644
--- a/include/types/hlua.h
+++ b/include/types/hlua.h
@@ -4,6 +4,9 @@
 #include <lua.h>
 
 #define CLASS_CORE     "Core"
+#define CLASS_TXN      "TXN"
+
+struct session;
 
 enum hlua_state {
 	HLUA_STOP = 0,
@@ -49,4 +52,15 @@
 	int function_ref;
 };
 
+/* This struct contains the pointer provided on the most
+ * of internal HAProxy calls during the processing of
+ * rules, converters and sample-fetches. This struct is
+ * associated with the lua object called "TXN".
+ */
+struct hlua_txn {
+	struct session *s;
+	struct proxy *p;
+	void *l7;
+};
+
 #endif /* _TYPES_HLUA_H */