If SQLITE_MAX_VARIABLE_NUMBER (default = 999) is changed at compile time
to be greater than SQLITE_MAX_COLUMN (default = 2000), which Debian does
by setting the former to 250000, Django raised an exception on queries
containing more than 2000 parameters when DEBUG = True.

This new technique is more straightforward and compatible with test
parallelization, where the effective database connection settings no
longer match settings.DATABASES.

Found cases where testing code was doing

    try:
        whatever
    except (some excption type):
        self.fail("exception shouldn't be thrown")

replaced it with just

    whatever

as this makes the unexpected errors easier to debug, and the tests
would fail just as much and aren't rendered less readable.

Thanks Markus Holtermann for review

The primary use case is to interact with a third-party database (not
primarily managed by Django) that doesn't support time zones and where
datetimes are stored in local time when USE_TZ is True.

Configuring a PostgreSQL database with the TIME_ZONE option while USE_TZ
is False used to result in silent data corruption. Now this is an error.

Thanks Carl Meyer and Tim Graham for the reviews.

Field.rel is now deprecated. Rel objects have now also remote_field
attribute. This means that self == self.remote_field.remote_field.

In addition, made the Rel objects a bit more like Field objects. Still,
marked ManyToManyFields as null=True.

Thanks to Russell Keith-Magee for mentoring this Google Summer of
Code 2014 project and everyone else who helped with the patch!

Thanks Daniel Hahler for the report and Tim Graham for the review.

Also used schema editor in migrate to sync unmigrated apps (sync_apps).
Refs #22340. Thanks Tim Graham for the review.

Each TestCase is also now wrapped in a class-wide transaction.