rename thread to worker_thread for clarity

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
use schedule_work to assign work to worker

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

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
select thread for message and work in constant time

Link: https://github.com/GeneAssembly/biosal/issues/36
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>