Optimize small FetcherData statistics

Each call to the SubmissionsList::query takes O(n) time. Suppose that we would like to run it for each contest, for each round, and for each round task to get these statistics: total number of problems, number of problems completed, total score, etc. It would take O(n^2) then what's bizarre.

But it's possible to be done in O(n) --- we traverse through the list of submissions once and simultaneously write all data to the FetcherData objects.