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
Actually Special:SMWAdmin is accessing the `job` table as well `		$row = $dbr->selectRow( 'job', '*', array( 'job_cmd' => 'SMW\RefreshJob' ), __METHOD__ );` and I wanted a class that collects all jobqueue related access (well currently only hosts the stats calls).