transport: use nonblocking transport by default

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
transport: a MPI message can have 0 bytes (this is allowed)

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
transport: disable nonblocking communication owing to a bug

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>