Slurm: schedule restarting jobs before other jobs in the queue

[mseitzer]

Restarting jobs are currently inserted at the back of the queue, thus need to wait until all other jobs in the queue are processed before being scheduled again. Slurm has a user priority feature that can influence the positioning of jobs in the queue. This could be used to favor restarting jobs over other jobs in the queue.

However, as stopping and inserting jobs in the queue is not atomic, Slurm might still dequeue and start another job before the restarting job is inserted in the queue.