PretouchTask should page-align the chunk size

PreTouchParallelChunkSize is a product option used to determine the chunk size for PretouchTask parallelism. Currently the option value is used directly for as chunk size. Because it's a product option its value could be anything within range. This can lead to adjacent chunks overlapping the same page. It would be better to use a page-aligned chunk size, so that if the start of the full range is page aligned (which is common) then all of the chunks (except perhaps the last) will have page-aligned bounds and won't have any pages in common with other chunks for the same task.