Allow use of Job Queue to process expirations

The attached patch allows for the use of the Job Queue module in d6 (Probably could use the queue system in d7 without too much work) to process cache expirations. The use case here could be if latency between the app servers and varnish is too long and you don't want to block the user actions that forces the expiration or could be a case where the number of urls to purge is large and so using the queue offloads the processing time to cron.

With cron called frequently this would keep content fresh while offloading expire processing at the point of the expire event.

With cron called infrequently this could be used to force a longer effective page lifetime for content that has a high frequency of expiration events while still being shorter than the full varnish maximum page lifetime.