assign work to available worker

before
argonnite.sh real    1m15.163s
scripts/run-argonnite-3.sh  6m20.883s

after
argonnite.sh real    0m57.103s
scripts/run-argonnite-3.sh  4m45.046s

Link: https://github.com/GeneAssembly/biosal/issues/235
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>