thorium_node: add function thorium_node_has_transport_congestion()

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_worker: use thorium_node_has_transport_congestion()

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_node: remove a useless message queue in the delivery path

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