thorium_actor: add action to enable the multiplexer

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
latency_probe: manually enable multiplexer

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
genomics: enable multiplexer in steps with numerous small messages

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_worker: use multiplexer when actor enabled multiplexer

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tests: test with 32 threads / node on Cetus

Link: https://github.com/GeneAssembly/biosal/issues/814
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>