Modify sinfo to operate in a federation

Add slurm_load_partitions2() function to route RPC to specific cluster
Add slurm_load_node2() and slurm_load_node_single2() functions to
  route requests to specific cluster in a federation
Modify srun to get node/partition information for each cluster in a
  federation at the same time using separate pthreads
Add sinfo sort by cluster name (--sort V)