Add SMW JobQueue statistics

Allow some insights about SMW related jobs and display them via `Special:Statistics`. `$smwgShowJobQueueStatistics` can be used to disable the display.

[0] http://wikimedia.7.x6.nabble.com/Re-Job-queue-affecting-semantic-queries-in-MW-1-22-tp5029433.html
Why not have methods such as `getUpdateJobCount`?