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
The message key is something specific to the presentation mechanism. A service class should not know about the presentation mechanism.