Eliminate O(n) loops in the TCP/IP stack

[nuta]

In smoltcp, the Kerla's TCP/IP implementation, there're some O(|# of sockets|) loops and it would lead to a scalability issue in the future:

https://github.com/smoltcp-rs/smoltcp/blob/8e6fc7fec85188a783ed80a25ee2159201aa74e8/src/iface/interface.rs#L2019

The priority is not that high, but it might be a good time to write our own TCP/IP protocol stack. I'd avoid to do so though.