Fixed #21134 -- Prevented queries in broken transactions.

Backport of 728548e4 from master.

Squashed commit of the following:

commit 63ddb271a44df389b2c302e421fc17b7f0529755
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Sep 29 22:51:00 2013 +0200

    Clarified interactions between atomic and exceptions.

commit 2899ec299228217c876ba3aa4024e523a41c8504
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Sep 22 22:45:32 2013 +0200

    Fixed TransactionManagementError in tests.

    Previous commit introduced an additional check to prevent running
    queries in transactions that will be rolled back, which triggered a few
    failures in the tests. In practice using transaction.atomic instead of
    the low-level savepoint APIs was enough to fix the problems.

commit 4a639b059ea80aeb78f7f160a7d4b9f609b9c238
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Tue Sep 24 22:24:17 2013 +0200

    Allowed nesting constraint_checks_disabled inside atomic.

    Since MySQL handles transactions loosely, this isn't a problem.

commit 2a4ab1cb6e83391ff7e25d08479e230ca564bfef
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sat Sep 21 18:43:12 2013 +0200

    Prevented running queries in transactions that will be rolled back.

    This avoids a counter-intuitive behavior in an edge case on databases
    with non-atomic transaction semantics.

    It prevents using savepoint_rollback() inside an atomic block without
    calling set_rollback(False) first, which is backwards-incompatible in
    tests.

    Refs #21134.

commit 8e3db393853c7ac64a445b66e57f3620a3fde7b0
Author: Aymeric Augustin <aymeric.augustin@m4x.org>
Date:   Sun Sep 22 22:14:17 2013 +0200

    Replaced manual savepoints by atomic blocks.

    This ensures the rollback flag is handled consistently in internal APIs.

tests/expressions/tests.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import, unicode_literals
 
 from django.core.exceptions import FieldError
 from django.db.models import F
+from django.db import transaction
 from django.test import TestCase
 from django.utils import six
 
@@ -185,11 +186,11 @@ class ExpressionsTests(TestCase):
             "foo",
         )
 
-        self.assertRaises(FieldError,
-            lambda: Company.objects.exclude(
-                ceo__firstname=F('point_of_contact__firstname')
-            ).update(name=F('point_of_contact__lastname'))
-        )
+        with transaction.atomic():
+            with self.assertRaises(FieldError):
+                Company.objects.exclude(
+                    ceo__firstname=F('point_of_contact__firstname')
+                ).update(name=F('point_of_contact__lastname'))
 
         # F expressions can be used to update attributes on single objects
         test_gmbh = Company.objects.get(name="Test GmbH")