lxd/db/node.go
func (n NodeInfo) IsOffline(threshold time.Duration) bool {
	return nodeIsOffline(threshold, n.Heartbeat)
}
func (n NodeInfo) IsOffline(threshold time.Duration) bool {
	return nodeIsOffline(threshold, n.Heartbeat)
}
// IsOffline returns true if the last successful heartbeat time of the node is
// older than the given threshold.