Better, faster, ptop_preindex

Also reduced chunk size to 100,000 from 250,000. This is a minor change we can tweak later if it makes things slower. 
I think that this won't be a problem, since it doesn't matter if we hit stuff twice, and run_ptop touches up any newer changes. (If run_ptop doesn't pick up at the seq cloudant was at when fast preindex started then that's a different bug.)