Don't automatically crash ns_heart if slow updater crashes.

Because seemingly there's no reason to do so. I created a supervisor
for both of these guys. If slow status updater occasionally crashes
because of (for instance) timeout, ns_heart will still be there and
will be able to sent at least stripped heartbeat.

Change-Id: I99fcccf9074ce817b9c226a327675e2b2ac0842e
Reviewed-on: http://review.couchbase.org/37136
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>