Background index builds should batch collection scan reads and inserts into the index

For every single document in a collection, background index builds retrieve a document, call saveState() (which resets the cursor) and restoreState() (which repositions it) after every single insert.

It would be more efficient to batch reads on the collection and inserts into the index so the read cursors are reset less often.

If we want to take advantage of read_once cursors, this will work around having the read the same page into cache when there are multiple documents in each page.