fix #3237 (closed)

Still todo:

• Intensive tests
• Some migration tool (see http://pgloader.io/). If you can come up with some methods that work reliably, please let me know. For now, I have this issue: https://github.com/dimitri/pgloader/issues/556#issuecomment-343777186
• Proper compilation and error support for the case where we compile with only sqlite3 but we provide a postgresql:// db_name value
• Document everything
• Make sure all the docker images can work with both methods