Add debug logging to catch edge cases in group sync

Related to Zendesk issue: https://gitlab.zendesk.com/agent/tickets/23090

We've managed to fix the issue outlined in the above ticket, but it feels like there's still some other edge case associated with update_existing_group_membership where some conditions aren't matched. In this case, it drops to add_new_members. At the end of that whole loop access_levels should reasonably be empty. Add some debugging logging to show the value of access_levels after each of the above methods is finished. If it's ever non-empty, we've encountered an edge case.