thorium: improve performance on Blue Gene/Q

latency_probe benchmark:

[thorium] node 499 SUMMARY Tick: 1352879 (31853.600000 Hz)
[thorium] node 499 EPOCH LOAD 30 s 2.13/15 (0.14) 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14
[thorium] node 499 EPOCH FUTURE_TIMELINE 30 s  0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
[thorium] node 499 EPOCH TRAFFIC_REDUCTION 30 s  0.77 0.77 0.77 0.76 0.77 0.75 0.77 0.77 0.77 0.77 0.77 0.77 0.76 0.76 0.77
[thorium] node 499 METRICS AliveActorCount: 1516 ByteCount: 1354031104 / 17163091968
[thorium] node 499 TRANSPORT ReceivedMessageCount: 922182 SentMessageCount: 922965 InboundThroughput: 31824.400000 messages / s OutboundThroughput: 31829.200000 messages / s
[thorium] node 499  MESSAGING BufferedInboundMessageCount: 0 BufferedOutboundMessageCountInRing: 1 MessageCountInTransport: 0

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