Draft: Add threadpool mappers for scheduler_reweight

Is better than serial code (100ms compared to 500ms in my test), but has one issue, which is some weights will be less optimal than before as the accumulation of weights will not happen between threads. This is why we use a uniform chunk to keep the work load from too many splits.

For any reasonable set of tasks this is a small issue, there is no effect measureable that I can see.