Add compaction times histogram to kvtimings

Change-Id: I255d0f66f38f8cf748903c992c85960430d1b29a
Reviewed-on: http://review.couchbase.org/34197
Reviewed-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>
Tested-by: Sundararaman Sridharan <sundar.sridharan@gmail.com>