thorium: fix memory leaks for worker outbound message buffers

There was a huge memory leak in the node.c code that interact with
message_multiplexer.c.

Link: https://github.com/GeneAssembly/biosal/issues/696
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: don't leak system inbound messages

Link: https://github.com/GeneAssembly/biosal/issues/698
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>