Update stats vbucket-seqno response to match spec

We will be sending both the high seqno and vbucket uuid in the
future. As a result we need a way to differentiate the two.

Change-Id: I3a318c835ae65d0747c46103d83dd0d0d7aa1c01
Reviewed-on: http://review.couchbase.org/30872
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>