message_multiplexer: use traffic reduction for success metric

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
multiplexer: track the number of messages for each channel

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
message_multiplexer: print traffic reduction for each channel

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