Made the AggregateInGallery job an adaptive batch job

Tested pretty thoroughly against a backup.  Rewound the GallerySettings cursor and had it replay the aggregation.  Tested that the aggregated stats increases (Packages and PackageRegistrations) matches the number of records that were rolled back.

Its adaptive batching will scale from 1000 to 100000 and in the sweet spot against a P1 database, it's processing 2000 stats per second.