simplification for spacesaver. before buckets were kept in an option structure called bucketsOption to defer their computation if not needed. upon a more careful look it turns out this is not very often. the idea that buckets does not have to be calculated in case of repeated merges of SSManys was the original main motivation (since this is what a hadoop reducer will do), but the merge depends on min which depends on buckets in almost all situations, so bucketsOption doesn't help us much here. bucketsOption can be hurtful if an SSMany gets serialized and deserialized, since in that case the None gets serialized and buckets has to be recomputed if needed after deserialization.