Refactor repository updates

[AMDmi3]

Repository fetch/parse routine became complicated, hard to extend and buggy. Refactor it to be more flexible, allow parallelization (#609), behavior tuning (allow fetch/allow refetch/honor not modified/honor update periods), make more tolerant to failures (#885, #937), more verbose (update statistics on the website should show detailed history), all in preparation to proper update daemon instead of single shot script (#551).