[server] Add a "scheduler" command to run repository scheduler This commands starts the repository scheduler as a standalone process that should complement a CubicWeb web instance running as a WSGI application. Added a log message in repository's shutdown method to help testing the command (i.e. make sure the method is called after the scheduler stopped). Related to #17057223.

[cwconfig] Do not instantiate a repo with a scheduler in repository method None of the caller of this method need the repository to have a scheduler and having a scheduler should be something that's explicitly requested.