Please make it possible for reduceExposure to parallelise over visits and arms

reduceExposure.py doesn't parallelise over all the input dataIds, although the processing is initially independent.  Please make it possible to run all visits/spectrographs/arms in parallel at least as far as a possible sequence point at full-focal-plane sky subtraction.  As I believe that there is not yet a need for such sequence points, it would be reasonable to delay that scatter/gather work until it's needed (and then probably we'll do it in gen3 middleware anyway)