Make "Index now" consume the items in the cron queue first

Since #946624: Use a cron queue for indexing got in, the „Index now“ function on indexes' „Status“ tabs has become a bit useless, as it will only index those items that weren't already queued for indexing in cron (which might still take days or weeks). The solution is probably to first consume the items from the cron queue, and only then request new ones from the data source controller.

Other solutions, patches or thoughts on this welcome!