Support for nested transactions (using savepoints) - with db.atomic()

[simonw]

For v4 I want to get our transactions story straight.

We have some surprising behavior there already. The table.transform() method starts its own transaction in a way that abruptly commits and existing transactions. There are a few other surprises tucked around too.

SQLite supports nested transactions using savepoints. We should support those too.

I think with db.atomic(): is good syntax for this, as also seen in Django and Peewee.