cache_google_appliance never clears older entries

I discovered on a production site that cache_google_appliance never clears out, it just keeps adding new entries. This becomes a serious performance impact when the table gets over 1,000,000 rows; we had 2,500,000 rows and the site was slowing to a crawl.

Attached is a simple patch that implements hook_flush_caches() that does the trick.