Caching mechanism overhaul & bug fix for duplicate pages for alphabetical indices

I have rewritten a lot of the code that caches indices. Normal caching has been removed, indices are now either not cached or cached aggressively. By setting cache lifetime to 0 (zero) caching will be disabled completely. I have also fixed a bug that could occur when caching alphabetical indices. It caused overviews of letters with the same equivalent in the modern Latin alphabet (E, É and Ë, for instance) to use the same cached page, meaning that only one of those pages would be used for all letters.

Fixed and committed to 6.x-2.x-dev.