thorium: improve multiplexer policy

./scripts/assembly_debugging/spate-saureus-fasta-gz-4x8-load.sh

8 KiB, 1 ms, 512 actors
DEBUG_MULTIPLEXER original_message_count 8648815 real_message_count 465392 (0.0538)
DEBUG_MULTIPLEXER original_message_count 8328393 real_message_count 478904 (0.0575)
DEBUG_MULTIPLEXER original_message_count 9845081 real_message_count 462847 (0.0470)
DEBUG_MULTIPLEXER original_message_count 10418093 real_message_count 476879 (0.0458)

real    1m14.019s
TIMER [Total] 1 minutes, 10.600639 seconds

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