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 <boisvert@anl.gov>
thorium_worker: first push in ring, if that fails, push in queue

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_worker: remove duplicate code that enqueues outbound messages

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