give all graph stores to walkers, and they will round robin on them

Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
rename dummy walker to unitig walker

Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
spawn more walker actors

Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
stop when all graph stores are empty

Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>