MB-11129 Ensure that all heartbeats have "slow" part.

Change-Id: I36d846d46f647fbf35c78c34981435957e8858c4
Reviewed-on: http://review.couchbase.org/37137
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>