thorium: use red-black tree to order multiplexing buffers in a timeline

Link: https://github.com/GeneAssembly/biosal/issues/807
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add comments for the lazy nature of buffer provisioning

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