Adapt the flusher's persistence function for UPR.

As the UPR requires a strict consistency in persisting dirty
items with respect to their by-seq numbers, the flusher should
not look up the hash table to persist the latest value for each
dirty item.

Change-Id: I83f8cca4e44cfdd6dc8da38daaa8dfc838a444bb
Reviewed-on: http://review.couchbase.org/33064
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>