We're currently sending whole NodeStatuses only to update a Node heartbeat. This a big source of traffic in the cluster, and one of the possible causes of 1000-node cluster failures. We need to extract heartbeat to a new API 'Heartbeat' object with timestamp and object reference only, and make Kubelet/NodeController to use this object instead of NodeStatus to determine health of the Node.

cc: @wojtek-t @lavalamp @bgrant0607 @brendandburns @smarterclayton @timothysc @davidopp @fgrzadkowski