Fix for the onPageTables formula

 - relies on HierarchyThreshold to find a good place to start the
   cleanup
 - validity of the formula was tested for ALL pagesizes ranging from 16
   byte to 32 Mbyte with ALL possible chunkSizes that result in a
   hierarchical page layout for the given pagesize.
 - Code that was used for validity testing can be found in  https://github.com/slizzered/mallocMC/commit/72614073f3fc882c32c7f13ac2fc205be316cefb
 - Code that demonstrated the bug in the first place: https://github.com/slizzered/mallocMC/commit/47f1754ba41e4cc5d876338499b201fbefefff7b
 - the previous comment in the code mentioned, that it might be possible to free
   only those areas that were used as metadata based on the previous
   chunkSize. This does NOT work: if the new metadata includes some
   parts that were previously used as payload-data, this parts will
   still be non-zero and the bug will occur again.
 - closes #70