Thanks Tim Graham and Markus Holtermann for review.

Thanks Josh Smeaton for Oracle fixes.

Thanks Mariusz Felisiak for review and Oracle assistance.
Thanks Tim Graham for review and writing docs.

When an error occurred during the cursor.execute statement, the cursor
is closed. This operation did not fail with client-side cursors. Now,
with server-side cursors, the close operation might fail (example
below). The original error should be raised, not the one raised by
cursor.close(), this is only clean-up code.

For example, one can attempt to create a named cursor for an invalid
query. psycopg will raise an error about the invalid query and the
server-side cursor will not be created on PostgreSQL. When the code
attempts to cursor.close(), it asks psycopg to close a cursor that was
not created. pyscopg raises a new error: psycopg2.OperationalError:
cursor "_django_curs_140365867840512_20" does not exist.

This prevents the base_field from sharing attributes with the one used
during migrations.

This particular test was sometimes failing when running the test suite
in parallel. The `id` was different depending on the order the tests
were run. The test was incorrectly comparing model primary keys rather
than ensuring they didn't change.

Thanks Peter Inglesby for the report and tests.

Thanks Florian Apolloner for contributing to this patch and
Collin Anderson, Markus Holtermann, and Tim Graham for review.

TestCase subclasses are wrapped in a transaction that prevents any data from
being persisted between test runs andi thus don't require limiting the tables
to be flushed to a subset of available apps like TransactionTestCase subclasses
do.

The initial implementation added support for PostgreSQL but it is also required
on Oracle (13b7f299).

Thanks Mariusz Felisiak for the foreign key retreival queries.

Regression in b52c7300.

Refs b3acdeee.

Thanks to Josh Smeaton for the idea of implementing server-side cursors
in PostgreSQL from the iterator method, and Anssi Kääriäinen and Kevin
Turner for their previous work. Also Simon Charette and Tim Graham for
review.

Regression in b52c7300.

Thanks Morgan Aubert for the test.

After refs #16614, integers might appear outside the WHERE clause.