Use spinlock instead of mutex for Threads and SplitPoint

Use spinlock instead of mutex for Threads and SplitPoint

It is reported to be defenitly faster with increasing
number of threads, we go from a +3.5% with 4 threads
to a +15% with 16 threads.

The only drawback is that now when testing with more
threads than physical available cores, the speed slows
down to a crawl. This is expected and was similar at what
we had setting the old sleepingThreads to false.

No functional change.