Improve fsfs-access-map tool: saturate at 64k accesses instead
of just 255 per 64kB cluster.  Tests show that we need some 
resolution for > 1000 hits / cluster as these are quite frequent.

Also, generate a scale bitmap that may be used as (the basis of)
a legend in e.g. presentations.

* tools/dev/fsfs-access-map.c
  (file_stats_t): mention that we record hits with 2 bytes now
  (word): new typedef
  (store_read_info,
   open_file,
   seek_file): update
  (select_color): update; refine color scale
  (write_bitmap_header): factored out from write_bitmap
  (write_bitmap): update
  (write_scale): new function
  (main): call the new function, too

git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1431268 13f79535-47bb-0310-9956-ffa450edef68