Reformat modul pagecache
Rename pagecache_it_DECLARE into pagecache_IT

Types with _T or _IT as suffix are template types.
Reformat module pagecache_impl
Support all pagesizes from 256Bytes to 1MB
Optimize size of pagecache_block_t

Size of pagecache_block_t optimized:

    sizeof(pagecache_block_t) == 32
    || sizeof(pagecache_block_t) == 64

This allows to use simple shift operations to access array entries.