some fixes for: https://issues.apache.org/jira/browse/AMQ-4487

Allow the maxProducersToAudit value to be set of Queue and QueueBrowser subscriptions which can help to workaround OOM errors on QueueBrowsers when there are more than the max number of producers audited.

git-svn-id: https://svn.apache.org/repos/asf/activemq/trunk@1489180 13f79535-47bb-0310-9956-ffa450edef68