use proc_lib to spawn slow_updater of ns_heart

I.e. so that if it dies we have some diagnostics

Change-Id: I73b721a2e1a561a4ee1e49a663540414a8cf5ac4
Reviewed-on: http://review.couchbase.org/30349
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>