[opensuse-buildservice] first draft of a dod implementation

Hi, here's a first (incomplete!) version of a patch which adds dod support to the backend. Before I implement the missing parts in bs_repserver I just want to know if this is the right way to do it (especially the parts in bs_sched and BSSolv.xs (the implementation in the repserver is just a quick hack to test the scheduler)). Note: currently it doesn't use the changes which were committed in r9280 because the scheduler rescans the repo directly if the repserver fetched new packages (although this might be problem in case the build finishes before the scheduler finishes the reposcan/processed the new event - I need to check if this can happen...). Feedback is welcome:) Marcus