- unused variable THD::con_root removed
- we don't need to bzero(&warn_root) just before init_alloc_root(&warn_root)
- we don't need to bzero(&transaction.mem_root) because we 
  bzero(&transaction) later in THD::THD
- we don't need to free thd->mem_root thd->transaction.mem_root
  in handle_one_connection because they are freed in THD::~THD
  which is called in end_thread later