thd_get_ha_data() can be used without a lock, but only from the
current thd thread, when calling from anoher thread it *must*
be protected by thd->LOCK_thd_data

* fix group commit code to take thd->LOCK_thd_data
* remove innobase_close_connection() from the innodb background thread,
  it's not needed after 87775402 and was failing the assert with
  current_thd==0