For review: issue 29 (slab stats' used_chunks) -- need backport?

[Dustin]

  I've put together a fix and test here:

      http://github.com/dustin/memcached/tree/issue_29

  In particular, does this test look valid?

      http://github.com/dustin/memcached/blob/issue_29/t/issue_29.t

  Should this be backported to 1.2.7?  It's mostly test and a small
change to how stats are computed, so that might make sense.