Add page_size option to RowProcessor to limit get_multi sizes

This option would be delivered to the RowProcessor implementation (if it's an instance of RowProcessor) to limit the number of documents fetched for a multi-get when dealing with many view results.

Currently if we get several thousand results in a single fetch, it means the include_docs handling with also try to do a get_multi on that many documents.

This will waste much memory and slow down the application.