thorium: prepare message in transport

Otherwise, the tracepoints in transport don't work

thorium_message:actor_send: (+0.007369926)
thorium_message:worker_send: (+0.000000985)
thorium_message:worker_enqueue_message: (+0.000001064)
thorium_message:worker_publish_message: (+0.000000658)
thorium_message:worker_pool_dequeue: (+0.000002508)
thorium_message:node_send: (+0.000001137)
thorium_message:node_send_system: (+0.000000786)
thorium_message:transport_send: (+0.000000586)
thorium_message:transport_receive: (+0.003211127) ****
thorium_message:node_receive: (+0.000001151)
thorium_message:node_dispatch_message: (+0.000000827)
thorium_message:worker_pool_enqueue: (+0.000001812)
thorium_message:worker_receive: (+0.000001183)
thorium_message:actor_receive: (+0.000003090)

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