[protocols/mdns] Remove Async<UdpSocket> and set the sockets as nonblocking

[gallegogt]

Description

In the previous version of the code the readable and writable functions were running constantly causing high CPU consumption constantly when using tokio as runtime.

Links to any relevant issues

• mdns: don't perform IO on main task #2591
• chat and chat-tokio examples: high CPU consumption (Ubuntu 20.04) #2675

Open Questions

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

Thanks for the report. I was operating under the impression that async-io is compatible with tokio. Is that not the case?

I don't think the patch proposed here is a valid solution. E.g. using a plain std UdpSocket does not give us wake-ups whenever the socket is ready to send or receive.

See also #2591 //CC @rkuhn.

[gallegogt]

Hi, thanks for you feedback, I'm going to close the PR, I still think it's a poor solution, I keep turning the problem around to find a better solution to the excessive CPU consumption that this module has.

I also think that using async-io when Tokyo is active is not a efficient solution since you are using two different reactors