Prevented clashes in the app registry.

Fixed #22790. Thanks timo for the report.

Refs #16723.

No test because this code is already deprecated (part of FastCGI support).

Stopped using the imp module on Python >= 3.3. Refs #21628.

Thanks guettli for the report.

``HttpRequest.build_absolute_uri()`` now correctly handles paths starting with ``//``.
``WSGIRequest`` now doesn't remove all the leading slashes either,
because ``http://test/server`` and http://test//server`` aren't the same thing
(RFC2396).

Thanks to SmileyChris for the initial patch.

Thanks santiagobasulto for the suggestion.

Prevented unlimited memory consumption when running background tasks
with DEBUG=True.

Thanks Rob, Alex, Baptiste, and others.

sphinx 1.2+ adds this text itself.

Fixed #20550 -- Added keepdb argument to destroy_test_db

Fixes #22605, #22735; also lays the ground for some other fixes.

Fixed #22725 - Migration.run_before does nothing

Forward-port of c9aedce0 from stable/1.7.x.

Forward-port of 1238f92c from stable/1.7.x.

Thanks Florian for the bug report, luyikei for the initial code patch, and
Bouke for the code review feedback.

This prevents silent data loss.

Thanks Aymeric Augustin for the initial patch and Loic Bistuer for the review.

Thanks clime for the report.

Previously this was conflated with another Oracle-specific behavior.