Fix on_commit execution order when callbacks make transactions

[craigds]

Backport of django/django@a5c8072

[coveralls]

Coverage decreased (-23.6%) to 76.364% when pulling cb27f9b on craigds:master into a3eabdd on carljm:master.

[craigds]

wow. unfortunately I'm a bit at a loss where to begin with that sqlite failure. Clearly it works in django 1.9 but I'm unclear why.

[carljm]

That error message still exists in Django today (https://github.com/django/django/blob/master/django/db/transaction.py#L153). Best I can suggest is to use pdb to trace through execution of that test in Django 1.10 (it wasn't added in 1.9), and also in 1.8 with django-transaction-hooks and this patch, to figure out what is different.