Improve abstraction in the MPI plugin API.

MPI plugins no long access structures in either srun or the slurmstepd.

Also, MPI plugins allocate state structures for each job step instance.

This allows the MPI plugins to be used in the SLURM API slurm_step_launch()
function.  It also allows slurm_step_launch() to be called multiple times
from within the same process.