thorium: reduce communication time for actor events

With ~ 1M data points

Before (median: 10.9378 ms)
> r=read.table('visitors.data-4x8.wait')
> quantile(r[,1], c(0.02, 0.10, 0.30, 0.50, 0.70, 0.90, 0.98))
      2%      10%      30%      50%      70%      90%      98%
    7663    56774  9523830 10937842 11946901 13080451 14172824

After: (median: 8.5423 ms)
> r=read.table('times')
> quantile(r[,1], c(0.02, 0.10, 0.30, 0.50, 0.70, 0.90, 0.98))
      2%      10%      30%      50%      70%      90%      98%
    7475    15501  6895831  8542374  9446864 10365630 11472052

Link: https://github.com/GeneAssembly/biosal/issues/751
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>