thorium: fix bug when an actor receives a message after dying

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_actor: add memory fence when changing assigned worker

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_worker: use YES and NO for easier reading

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