Added BaseDatabaseWrapper.ensure_connection.

This API is useful because autocommit cannot be managed without an open
connection.
Removed _enter/_leave_transaction_management.

The goal is to make all databases share a common, autocommit-based,
implementation.
Enabled database-level autocommit for all backends.

This has the same backwards-incompatibility consequences as those
described for PostgreSQL in the previous email.
Deprecated transaction.commit/rollback_unless_managed.

Since "unless managed" now means "if database-level autocommit",
committing or rolling back doesn't have any effect.

Restored transactional integrity in a few places that relied on
automatically-started transactions with a transitory API.
Documented database-level autocommit.
Deprecated transaction.is_managed().

It's synchronized with the autocommit flag.
Moved standard SQL for savepoints in the base backend.

These methods are only called when uses_savepoints = True anyway.
Refactored implementation of savepoints.

Prepared for using savepoints within transactions in autocommit mode.
Added support for savepoints in SQLite.
Implemented an 'atomic' decorator and context manager.

Currently it only works in autocommit mode.

Based on @xact by Christophe Pettus.
Made atomic usable inside the legacy transaction management.
Added some assertions to enforce the atomicity of atomic.
Deprecated the old transaction management APIs.