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.

Previously, if a database request spanned a related object manager, the
first manager encountered would cause a request to the router, and this
would bind all subsequent queries to the same database returned by the
router. Unfortunately, the first router query would be performed using
a read request to the router, resulting in bad routing information being
used if the subsequent query was actually a write.

This change defers the call to the router until the final query is acutally
made.

It includes a small *BACKWARDS INCOMPATIBILITY* on an edge case - see the
release notes for details.

Thanks to Paul Collins (@paulcollinsiii) for the excellent debugging
work and patch.

Refs #18091.

The issue was reported against 1.3.x but has been fixed since.

Improve tests to cover slicing scenarios that could be handled in unique
ways by 3rd party database backends.

ChoiceFieldRenderer was not rendering nested choices. Added recursion
to ChoiceFieldRenderer to take nested choices and render them as
<ul>'s.

Added it to the settings reference. We used to have it in comment form
in our old, bigger default project template settings.py file.

Code had been added in e909ceae. Solves test suite failures observed
on Windows.

Thanks Michael Manfre for the report. Refs #20739.

The import was done from util instead of utils leading to
PendingDeprecationWarning.

Thanks to onlygoldi2201 for the report and to ramiro and apollo13
for the reviews.

Also fixed a py3 incompatibility.
Thanks to brutasse for the report.

I a word :-/

DatabaseCache uses raw cursors to bypass the ORM. This prevents it from
being used by database backends that require special handling of datetime
values.

There is no easy way to test this, so no tests added.

Thanks to EmilStenstrom for raising this, and to Chris Piwoński for all of the fixes and most of the tests.

A .annotate().select_related() query resulted in misaligned rows vs
columns for compiler.resolve_columns() method.

Report & patch by Michael Manfre.

The commit for #18333 missed quote_cache default value for *.
Refs #18333.

Thanks go to Michael Gebetsroither for pointing out this issue and help on
the patch.

This reverts commit aae5a96d.

This fix is no longer necessary, our pbkdf2 (see next commit) implementation
no longer rehashes the password every iteration.

Thanks marfire for the report.

Fixed #21152: Corrected small grammatical error in comment.

Changed 'there are no filtering' to 'there is no filtering'.