MB-12067: Add additional defragmenter_num_visited stat

Records how many items have been visited (considered for
defragmentation). Useful to measure this against
defragmenter_num_moved.

Change-Id: I56f3509645349b0ef482afcb9b68c3049154143b
Reviewed-on: http://review.couchbase.org/43052
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Dave Rigby <daver@couchbase.com>