Various improvements to "osv leak show":

1. Somewhat faster performance (but still slow).

2. Better report of progress (percent done). Previously, much of the`
   work of fetching the backtraces from the guest was actually delayed
   until sort time, so was wrongly attributed to the sort phase. Now
   the fetching phase takes most of the time, and percent of its
   progress is shown.

3. Due to popular request: sort leak records by size:

   Instead of outputting immediately each leak record (summary of all
   living allocations from a particular call chain), we now save them
   in memory, making it very easy to sort these records by any interesting
   criterion.
   In this patch, I sort them in decreasing order of total bytes - i.e.,
   the first record one sees is the one responsible for most allocated
   bytes.

   The additional sort takes only a fraction of a second, and makes the output
   of "osv leak show" much more useful.