Make pager more efficient by not generating (and running) a countQuery

Many moons ago I filed issue https://www.drupal.org/node/778050 again Drupal. That didn't go anywhere, because the pager was not pluggable and needed to be database-agnostic.

Happily, Backdrop only supports MySQL, so the pager changes should in theory Just Work™. This means that complicated queries no longer need custom count queries and the database doesn't need to run two queries each time a pager is displayed on the site.

I ran a lot of beanchmarks on the original issue and there was no performance degradation for small tables, and improved performance for large tables. You can see details at http://cafuego.net/2010/05/26/fast-paging-real-world