thorium_message: add a method to get the buffer signature

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tracing: add buffer signature in thorium_actor_send LTTng event

Link: https://github.com/GeneAssembly/biosal/issues/833
Link: http://lists.cels.anl.gov/pipermail/biosal/2014-December/000139.html
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>