Add maintenance status

[Olivier Sallou]

If in a maintenance status, scheduler should stop sending tasks No impact on watcher that should continue to check jobs.

This is needed to restart/update/upgrade software. A script would set maintenance on, wait for the end of the jobs then stop the scheduler.

the watcher can be silently stop/restarted with no impact.

[Olivier Sallou]

Manage maintenance status, stop scheduling jobs if maintenance of system is on, closes #31

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