thorium: set mpi1_pt2pt_nonblocking as default transport subsystem

the memory leak reported in #783 is no longer reproducible.

Link: https://github.com/GeneAssembly/biosal/issues/782
Link: https://github.com/GeneAssembly/biosal/issues/783
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>