MEDIUM: lua: remove Lua struct from session, and allocate it with memory pools

This patch use memory pools for allocating the Lua struct. This
save 128B of memory in the session if the Lua is unused.
diff --git a/include/types/stream.h b/include/types/stream.h
index 26e8dd5..227b0ff 100644
--- a/include/types/stream.h
+++ b/include/types/stream.h
@@ -165,7 +165,7 @@
 	/* These two pointers are used to resume the execution of the rule lists. */
 	struct list *current_rule_list;         /* this is used to store the current executed rule list. */
 	void *current_rule;                     /* this is used to store the current rule to be resumed. */
-	struct hlua hlua;                       /* lua runtime context */
+	struct hlua *hlua;                      /* lua runtime context */
 };
 
 #endif /* _TYPES_STREAM_H */