Minor task manager enhancements

[olefirenque]

I would like to argue about the note "possible data-races can make it even more random":

• data-races are likely to disrupt the random distribution
• I suppose single random object still can cause contention by cache line collisions

I would also like to propose some heuristic for stealing single task. I think since it barely contends with other threads, it might be useful to do a few more iterations.

P.S: Are there any convenient deterministic benchmarks for comparing performance with such minor changes in xray?