pub fn new(shard_update: Box<ResolverUpdateSharder<T>>) -> Self {
        Self {
            subchannel_child_map: HashMap::default(),
            children: Vec::default(),
            shard_update,
        }
    }
Creates a new ChildManager LB policy.  shard_update is called whenever a
resolver_update operation occurs.
grpc/src/client/load_balancing/child_manager.rs