Enabled autocommit for PostgreSQL in all circumstances.

For users who didn't activate autocommit in their database options, this
is backwards-incompatible in "non-managed" aka "auto" transaction state.

This is the default transaction state, and it's also the state that
results from enter_transaction_management(False). This state now uses
database-level autocommit instead of ORM-level autocommit. Queries that
bypass the ORM are now committed immediately and no longer need to call
transaction.commit_unless_managed().

Technically speaking, this is a regression, and it will be fixed at a a
later point.

This doesn't change anything for users who enabled autocommit in OPTIONS.
It doesn't change anything either for update() and delete() ORM calls,
since they temporarily enable transaction management. It might change
the behavior of other ORM operations (multi-table saves?) -- to be
confirmed, and if necessary, fixed in a later commit.