Add ability to pause worker scheduling

[benoit74]

Currently when one want the manager to stop starting new task, it is needed to completely stop the container.

This has many drawbacks:

• worker seems dead in the UI while it is just paused from starting new tasks
• should the machine reboot or docker restart, the manager will restart automatically and begin again to accept new task
• all other duties of the manager are not performed anymore (e.g. atm cancelling tasks that need to be)

I think it would help to create a new status for the worker : cordon (to reuse k8s term) or scheduling_disabled

In this status the worker would just stop starting new tasks but will continue all the rest of its duties.

This should probably be configured in /etc/zimfarm.config (needing a manager restart, but this is not a problem)

This status should be reported during check-in, stored in DB, displayed in UI.

[rgaudin]

Can be done by toggling the OFFLINERS conf to empty in that same file in the mean time ; or by setting SELFISH (not exactly equivalent but would work for most cases)