tracing: add LTTng tracepoint thorium_actor:send

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium_actor: add instrumentation for thorium_actor:send

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tracing: add script to trace a visitor actor

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