Track session state only when required

The protocol should not track the session state as the parsing is quite
expensive with the current code. This change is a workaround that enables
the parsing only when required. A proper way to handle this would be to do
all the response processing in one place thus avoiding the duplication of
work.

server/modules/routing/readwritesplit/rwsplit_route_stmt.cc

@@ -280,6 +280,11 @@ bool RWSplitSession::route_single_stmt(GWBUF* querybuf)
         }
         else if (TARGET_IS_MASTER(route_target))
         {
+            if (m_config.causal_reads)
+            {
+                gwbuf_set_type(querybuf, GWBUF_TYPE_TRACK_STATE);
+            }
+
             succp = handle_master_is_target(&target);
 
             if (!succp && should_migrate_trx(target))