Reintroduced transaction.commit_on_success() but wrapping only each new Attachment object creation.
Also, since Django is bad at dealing with threads in general, and get_or_create() on mysql is buggy,
restructured the threaded class to use the Instance pk to find the parent, then explicit look for
and then create (if DoesNotExist is triggered) each attachement object.
The queue and daemon thread pools from the previous version is retained.