MB-11107: The amount of processed bytes in an upr consumer should be atomic

We update this variable from different threads and as a reuslt need to make
it atomic to prevent concurrency issue. This fixes an issue where the
amount of acked bytes on the consumer did not match up with the producer.

Change-Id: I991ad51826be97404aaf4b63a9d616ffde62508a
Reviewed-on: http://review.couchbase.org/37623
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>