(Bug 52642) Add query duration as meta data option

QueryDuration is designed to be an opt-in meta data (by default this options
is disabled) and enabling it requires to run update.php.

$GLOBALS['smwgQueryProfiler'] = array(
    'smwgQueryDurationEnabled' => true, // enabled
);
Merge pull request #30 from SemanticMediaWiki/queryprofiler-duration

(Bug 52642) Add query duration as meta data option