use a power of 2 in the ring

Before: 0m28.518s
After: 0m28.183s

Link: http://psy-lob-saw.blogspot.com/2013/03/single-producerconsumer-lock-free-queue.html
Link: https://github.com/GeneAssembly/biosal/issues/313
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>