Adding batch support to the rebuild queue process both on UI and Drush.
This patch adds patch support by building on top of the patch provided in comment #5 with a few differences.
First, it adds supports to variants as is the case with the current release version of sitemap (the patch in 5 pulls the variants from the manager so ends up building all regardless of parameters provided in drush).
Second it isolates only the batch changes, the rebuild queue decoupling has moved to another merge request.