MB-11267: Change total_unacked_bytes to unacked_bytes

When we use the word total we are giving off the impression that
this is a cumulative stat and it is not.

Change-Id: Ie8b275319181f7010ed6e1ca4b091387d848df2a
Reviewed-on: http://review.couchbase.org/37853
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>