Add vbucket-seqno stats to vbucket-details

We should include these stats in the vbucket details stats as well
since vbucket-seqno is really only mean to be used internally for
clients that only want to get the bare minimum needed to make dcp
connections.

Change-Id: Ib95b80f8ec7afb37b146d6932e7c722da9d917c4
Reviewed-on: http://review.couchbase.org/39968
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Reviewed-by: Sundararaman Sridharan <sundar@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>