Allowed checkout as alias for usevar since pretty clear
Redo accounts example using STM, experimental form

In particular, as well as using it the way it's designed to be also using
it in a way that allows a potentially cleaner way.
Progressive example usage of the STM code

Overview of progression:

 * accounts-stm-1.py - Simplest (least finesse) use of the STM code
 * accounts-stm-2.py - Wrap up retrying in a decorator - simplifies the
                       code, and removes a probable cause of error.
 * accounts-stm-3.py - Limit number of retries to avoid starvation.
 * accounts-stm-4.py - Use a timeout instead/as alternative.