xmlsitemap_node_node_update() should check for a queue item before creating a new one

Use case:

• The same node is updated numerous times per day (either manually or by Feeds import)
• xmlsitemap_node_node_update() creates a new queue item each time the node is updated
• xmlsitemap_node_cron doesn't have a chance to process and delete the queue items for whatever reason (error, cron timeout, etc)
• Numerous queue items persist in the queue table for the same nodes, which creates a queue backlog which thwarts sitemap.xml from updating properly

After installing the very helpful queue_ui module, we discovered over 300,000 queue items for only a few hundred nodes of a particular content type. Yes, we need to figure out the underlying issue, but xmlsitemap doesn't need to create a new queue item for existing, unprocessed items.

I propose checking for a queue item before creating it.

Patch is on its way . . .