Account for UPR_EXPIRATION in upr-producer's getNextItem()

Change-Id: Ib31c01617398c35ebbce56784835af8a49b7e260
Reviewed-on: http://review.couchbase.org/38345
Reviewed-by: Michael Wiederhold <mike@couchbase.com>
Tested-by: abhinav dangeti <abhinav@couchbase.com>