MB-11459: Properly set high seqno in couchkvstore

We missed two places where we should update the cached version of
the vbucket high seqno.

Change-Id: I0b7e32348948814cb54d93feb5d9d78c8d478196
Reviewed-on: http://review.couchbase.org/38774
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>