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>