[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.