MB-11545 failover safeness level calculation for UPR

Change-Id: Id8f473c494fa2d5bc28e41ae9b6f26a58882d521
Reviewed-on: http://review.couchbase.org/38815
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>