latency_probe: enable multiplexer for target actors
latency_probe: enable multiplexer for target actors

All the tests for the past few days were useless since
the target actors were not using the multiplexer.

New throughput on the development machine:

PERFORMANCE_COUNTER computation-throughput = 3064693.241291 messages / s

Signed-off-by: Sébastien Boisvert <1a4d2e25eb5ba040b6ad5779a730a1a8a0ff9bc5@anl.gov>

GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal,GeneAssembly/biosal,sebhtml/biosal,sebhtml/biosal,sebhtml/biosal,GeneAssembly/biosal,GeneAssembly/biosal,sebhtml/biosal,GeneAssembly/biosal,sebhtml/biosal