MB-10382 adopt new format of vbucket-seqno stat

Change-Id: I7167369eec4cda3bba151f18698f75101fb88126
Reviewed-on: http://review.couchbase.org/34651
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>