SPEED: Add support for per-chunk/per-future seeds

[HenrikBengtsson]

(from an old note of mine)

Add support for generating seeds per chunk (=future), i addition to per element as now. Although not invariant to the number of workers, and therefore not numerically reproducible if nbrOfWorkers() vary, this will still be statistically sound. The advantage is performance - the number of seeds that need to be pre-generated will be significantly lower when there's a large number of elements.

What should the API be, i.e. how to specify that we want it per chunk without introducing more arguments? Would future.seed = "per chunk" vs future.seed = "per element" (= TRUE as now). Then what about seed = <numeric> for specifying an initial seed (which now gives "per element" seeds)? Hmm...