Make it possible to override USE_TZ & TIME_ZONE on a per-database basis

[Aymeric Augustin]

When integrating with a legacy database, datetimes are most often encoded in local time. This doesn't work well with projects where USE_TZ = True.

The configuration for each database backend should accept a TIME_ZONE option to enforce interpretation of datetimes stored in that database as naive values in that time zone.

Setting this option when USE_TZ = False could either be an error or perform a conversion to the global default TIME_ZONE. The latter may be too complicated to be worth the effort.

[Aymeric Augustin]

At least for SQLite, this will require getting rid of the ​global converters as there is no way to register converters per connection.

I'll try to replace the converters, which are quite messy anyway, with an implementation of DateTimeField.from_db_value.

[Aymeric Augustin]

DatabaseOperations.convert_datetimefield_value seems to be a better match for what I'm trying to achieve.

See e9103402c0fa873aea58a6a11dba510cd308cb84.

[Aymeric Augustin]

This will required getting rid of the global converters and adapters for datetimes, because they're hardcoded to use UTC regardless of the database connection being used.

Proposal: ​https://groups.google.com/d/msg/django-developers/Ci_cs0KTagM/DchJIB8-vRsJ

[Aymeric Augustin]

PR: ​https://github.com/django/django/pull/4601

[Aymeric Augustin <aymeric.augustin@…>]

In ec18657:

Removed global timezone-aware datetime converters.

Refs #23820.

[Aymeric Augustin <aymeric.augustin@…>]

In d9521f6:

Removed global timezone-aware datetime adapters.

Refs #23820.

Fixed #19738.

Refs #17755. In order not to introduce a regression for raw queries,
parameters are passed through the connection.ops.value_to_db_* methods,
depending on their type.

[Aymeric Augustin <aymeric.augustin@…>]

In ed83881:

Fixed #23820 -- Supported per-database time zone.

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.