Rework blog post indexing

[rku]

Currently, all postings are indexed per request. This has some disadvantages regarding scalability. Therefore, the blog post indexing mechanism should be refurbished.

[horazont]

However, this is not as trivial as it might seem on the first glance. We probably cannot rely on filesystem timestamps on all supported platforms (we'd have to check that) or at least we would have to check way too many files on each request, even if we only compared timestamps (which might be very well what we're doing right now).

inotify would be an option, requires a lot of managment and adds platform restrictions.