Implement possibility to follow job log in arcctl (aka tail -f approach)

Now you can nicely monitor the job log progressing in the tail -f manner from arcctl CLI, e.g.:

arcctl job log gMyNDmOu3RtnjwO5upha6lOqABFKDmABFKDmDYMKDmscFKDmHgUHVo --follow

Useful for testing and live demos.