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/src/stream.c b/src/stream.c
index 298830d..522cd34 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -221,8 +221,7 @@
 	s->res.analyse_exp = TICK_ETERNITY;
 
 	s->txn = NULL;
-
-	HLUA_INIT(&s->hlua);
+	s->hlua = NULL;
 
 	if (flt_stream_init(s) < 0 || flt_stream_start(s) < 0)
 		goto out_fail_accept;
@@ -299,7 +298,8 @@
 		offer_buffers(NULL, tasks_run_queue + applets_active_queue);
 	}
 
-	hlua_ctx_destroy(&s->hlua);
+	hlua_ctx_destroy(s->hlua);
+	s->hlua = NULL;
 	if (s->txn)
 		http_end_txn(s);