MB-10368: Get number of items from UPR

Instead of getting the total number of items from the database files,
use some UPR stat to get that information.

Change-Id: I86db1dc22f864de0e90a894967899587deab8e49
Reviewed-on: http://review.couchbase.org/34439
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Tested-by: Volker Mische <volker.mische@gmail.com>
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>