thorium: actually save reply messages in message cache

Reviewed-by: Élénie Godzaridis <telomerika@gmail.com>
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add debug message when cleaning the message cache

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