final code change for dlenevs review.
- Don't use pruning in lock_count().
- Don't use pruning in store_lock() if not owning thd.
- Renamed is_fields_used_in_trigger to
  is_fields_updated_in_trigger() and check if they
  may be updated.
- moved out mark_fields_used(TRG_EVENT_UPDATE)
  from mark_columns_needed_for_update().
  And reverted the changed call order. And call
  mark_fields_used(TRG_EVENT_UPDATE) instead.