[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.

mock
psycopg2
ldap3 < 2
cubicweb-basket
cubicweb-card
cubicweb-comment
cubicweb-file
cubicweb-localperms
cubicweb-tag