Add SMW JobQueue statistics

Allows some insight 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
> I wanted a class that collects all jobqueue related access

You can always make a façade if there really is need for that. What you are doing now is optimizing, in a way I find somewhat dubious, for a situation which we do not currently have.