latency_probe: display number of send and received messages at the end

Link: https://github.com/GeneAssembly/biosal/issues/794
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
latency_probe: use a random start and round-robin selection

The code in genomics/assembly/unitigs also uses this design for
fairness.

Link: https://github.com/GeneAssembly/biosal/issues/794
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>