Drive

Starting support for Google Drive.
But there is no server-side component to the Google Drive implementation, is there? How does this flag ever have an effect? I still don't think any private keys should exist in the model.

In the existing code, making a checkpoint doesn't include a save operation, it just records the most recent save as a checkpoint to remember. If there's no such thing as autosave without checkpoint on Google Drive, then you should probably just make checkpoint a no-op.