"Batch" together delete (and possibly update) operations in seeding

MigrationsModelDiffer.GetDataOperations "batches" together multiple inserts to the same table, so that the scaffolded migration code looks much more readable (one insert operation with a big array instead of lots of single-row insert operations).

We could do the same for deletes, and possibly even for updates, as long as the column list is the same (not sure it's worth it).