Implement proper support for minimum cache lifetime

Currently the memcache module's only support for the minimum cache lifetime setting on the Drupal performance page actually enforces a MAXIMUM cache lifetime. I consider this a rather annoying bug, though I'm filing this as a feature request as the needed feature is proper support for minimum cache lifetime.

The attached patch is for the 6.x-1.x-dev version of this module -- it may apply cleanly to the 5.x version too, if not I will back port it as I want to see this feature in both versions of the module.

The minimum cache lifetime is implemented by tracking a global timestamp for each cache table -- instead of "flushing" the table, we update the timestamp. We then compare cache items with this global timestamp when we do a cache_get, and if the cached item is more than the "minimum cache lifetime" older than the flush timestamp, we return 0 to cause the item to be regenerated. As with Drupal core, we also make sure that the anonymous person caching the cache flush sees an updated version of the website -- however we track this in the $_SESSION rather than in $user->cache, as we need to track one timestamp for each table.

This improves upon the core minimum cache lifetime in the following ways: 1) it implements lazy cache invalidation (no DELETE * query), and 2) if, for example, the minimum cache lifetime is set to 10 minutes and the cache was last flushed 30 minutes ago, then the cache will be flushed immediately when the next content is posted. (With the core implementation, the cache isn't flushed for another 10 minutes, which imo is silly as we've already gone 30 minutes without a flush.) If there's been no cache flush for 30 minutes, the minimum cache lifetime is set to 10 minutes, and two comments are posted one after the other, the first will flush the cache, the second will lead to the cache "flushing" 10 minutes later properly enforcing the minimum cache lifetime.

This patch was sponsored by ParentsClick/ Lifetime Digital