thorium_actor: add template for ACTION_SPAWN_MANY

Link: https://github.com/GeneAssembly/biosal/issues/635
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_actor: fix typo

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: implement thorium_actor_spawn_many()

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