Add ability to limit job's leaf switch count

Add the ability for a user to limit the number of leaf switches in a job's
allocation using the --switch option of salloc, sbatch and srun. There is
also a new SchedulerParameters value of max_switch_wait, which a SLURM
administrator can used to set a maximum job delay and prevent a user job
from blocking lower priority jobs for too long. Based on work by Rod
Schultz, Bull.