Fix to #473, http://bugs.skysql.com/show_bug.cgi?id=473
Several memory issues. Read the code.
In general one-off hint is only added to gwbuf and freed in gwbuf_free. Stacked hint is copied to stack and to gwbuf. gwbuf is freed after routing but stacked hint is freed either in stop or when session is closed. All this applies to named hint as well except that in addition, it has one more copy in named hint struct which is emptied when session is closed.
