mpi: use MPI_Test rather than MPI_Testany or MPI_Testsome
mpi: use MPI_Test rather than MPI_Testany or MPI_Testsome

MPI_Testany or MPI_Testsome does not follow message order -- unless we
rotate the request array, which is too much trouble. Use MPI_Test plus
rotating the request follows the message order.

If we don't follow message order, during heavy message rate, message at
the last slot of multi-recv buffer will be delayed as long as new
message gets in, potentially messing up payload match.

StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion,StanfordLegion/legion