clean post cache after publish_future_post event is fired and post is changed

[hailin]

After lengthy debugging, it turns out that in many cases where "Missed Schedule" is diplayed, the root cause is that we didn't clean the post cache after the post status is changed from future to publish.

When any portion of post is changed, we should invalidate the post cache.

[hailin]

updated patch

[ryan]

That triggers the save_post action which calls _save_post_hook() which does the cache flush.