Near Procedure Call.
let npc = NPC({message in
// send message to near npc
})
npc.on("ping") { param, notify, reply in
// reply content
reply("pong", nil)
// return a cancel function which could be nil.
return nil
}
n0.deliver("ping", param: nil, timeout: 0, onNotify: {param in
// param is notification
}, onReply:{param, error in
//param is repication
//error is error-replication
})