Implemented promotion of repack requests from Pending to ToExpand
This promotion is controlled so that only a limited number a requests are in the state ToExpand or Starting at any point in time. This ensures both the availabality of repack file requests to system while preventing an explosion of file level requests. Created a one-round popping from the container (algorithms) with status switching. - Used for repack requests switching from pendig to to expand Added ElementStatus to algorithms. Implemented promotion interface in Scheduler and OstoreDb. The actual decision is taken at the Scheduler level. The function itself is called by the RepackRequestManager. Promotion is tested in a unit test. Various code maintenance: Switched to "using"-based constructor inheritance. Fixed privacy of function in cta::range.