calculate ratio in multiplexer and improve the performance in graph traversal

Old:
1 KiB, 512 us
1 minutes, 25.049934 seconds

New:
1 KiB, 1 ms
1 minutes, 26.011803 seconds
DEBUG_MULTIPLEXER original_message_count 9466124 real_message_count 964310 (0.10)

1 KiB, 2 ms
1 minutes, 22.833992 seconds
DEBUG_MULTIPLEXER original_message_count 9521309 real_message_count 837602 (0.0880)

Link: https://github.com/GeneAssembly/biosal/issues/675
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: skip particular actions in multiplexer

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: separate the multiplexer mechanism from its policy

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