MB-10472: Change default fake UPR server behavior

Change the fake UPR server to return only half of the items when only
the persisted items are requested. This will be a common behaviour in
Couchbase, hence it makes sense to run all unit tests with this setup.

Change-Id: Icb5b0cf70f489c45d0e0a909f1bc1635741de5de
Reviewed-on: http://review.couchbase.org/35907
Reviewed-by: Filipe David Borba Manana <fdmanana@gmail.com>
Tested-by: buildbot <build@couchbase.com>