Refactor and test getPageCount

Use panic in case of unrecoverable errors - there's nothing we can do in case of App Engine failures here.
Test basic functionality of page counting.
Make sure post count will eventually update.

Expire the cached post count after an hour, so that possible miscounts (because store operations haven't applied yet) are eventually corrected.
Do not update the post count just when the post was stored, so that there is more time for datastore to apply the change before the next load.
Test loading and storing of posts.