evolve: drop useless return This return statement is never reached. This a actually a good new because its variable was undefined. We remove it. (caught by laurent using pylint)

evolve: add missing import (caught by pylint)

evolve: improve transaction related code to use the lock module Refactored some of the places where we are using locking in evolve to use the lock module and write less code. This should also fix a couple of bug where tr could be None at release time.