Fixed #21803 -- Added support for post-commit callbacks

Made it possible to register and run callbacks after a database
transaction is committed with the `transaction.on_commit()` function.

This patch is heavily based on Carl Meyers django-transaction-hooks
<https://django-transaction-hooks.readthedocs.org/>. Thanks to
Aymeric Augustin, Carl Meyer, and Tim Graham for review and feedback.

tests/transaction_hooks/models.py

@@ -0,0 +1,10 @@
+from django.db import models
+from django.utils.encoding import python_2_unicode_compatible
+
+
+@python_2_unicode_compatible
+class Thing(models.Model):
+    num = models.IntegerField()
+
+    def __str__(self):
+        return "Thing %d" % self.num