Manual prescan (autoscan) shouldn't be run as a separate job.

This pull request extends #907. Since single queue is maintained by more than one worker, a manual prescan (autoscan) should be scheduled in the same job as the 'main' scan - otherwise there's a risk that the 'main' scan will be launched before prescan.

And also, all the functions/methods that queue some sort of a scan instead performing it directly, should be named accordingly (I propose ``queue_`` prefix for that purpose) - otherwise such naming scheme is quite misleading.