- Initialize a pool of bucket zones so that we waste less space on zones that

 - Initialize a pool of bucket zones so that we waste less space on zones that
   don't cache as many items.
 - Introduce the bucket_alloc(), bucket_free() functions to wrap bucket
   allocation.  These functions select the appropriate bucket zone to
   allocate from or free to.
 - Rename ub_ptr to ub_cnt to reflect a change in its use.  ub_cnt now reflects
   the count of free items in the bucket.  This gets rid of many unnatural
   subtractions by 1 throughout the code.
 - Add ub_entries which reflects the number of entries possibly held in a
   bucket.