Function is_responsible 

pub async fn is_responsible(
    self_id: &PeerId,
    key: &XorName,
    p2p_node: &Arc<P2PNode>,
    responsibility_width: usize,
) -> bool

Check if this node is within a caller-supplied closest-peer width for key K.

Returns true if self_id is among the responsibility_width nearest peers to K in SelfInclusiveRT.