Use the read-write KV instance to get the last persisted seqno.

The read-write KV instance is always synchronized with the last
persisted seqno.

Change-Id: I312f5af31f247d9520621caa02f3467021dc8bd6
Reviewed-on: http://review.couchbase.org/40022
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>