Add KCP interface support

[ghost]

KCP is a fast and reliable ARQ protocol. KCP can provides faster connection in some environment.
I test this code, it works good.

[Vort]

I wonder how you will build Windows 7 compatible binaries with this change.

[ghost]

I wonder how you will build Windows 7 compatible binaries with this change.

It's a problem that has to be faced, but Windows 7 Microsoft has stopped supporting it, and it's going to go.
kcp-go requires Go 1.21 or higher, it will indicate that Yggdrasil no longer supports Windows 7.
But this pull request can be delayed, until the author want to merge it.

[NikitaPuzyryov]

Fwiw @neilalexander had to say this in the Matrix room:

That's also a rather frightening list of new transitive dependencies...

[ghost]

I have a solution make Yggdrasil run on Windows 7. Use the old version of kcp-go. Add it to go.mod

replace github.com/xtaci/kcp-go/v5 => github.com/xtaci/kcp-go/v5 v5.6.1

This is the last compatible version of Go 1.20

[neilalexander]

Thanks for raising the PR but I don't know that we'll accept it at this time. I'd also invite @Arceliar's opinion too.

We are concerned mostly about three things:

1. KCP has quite a lot of tuneables that could potentially make it worse than TCP or QUIC congestion control — we don't want to expose those knobs to users but we don't know the best answers either;
2. Our time is already somewhat limited and this is adding to our maintenance overhead when the vast majority of the network is using TLS/TCP;
3. There's a lot of transitive dependencies in this library that suddenly we have to care about regarding security vulnerabilities & fixes.

If we were going to accept this, we'd need some convincing with concrete evidence (perhaps plotted on graphs) to prove that it performs well enough or better than the TLS/TCP and QUIC modes that we already have or that it satisfies some use-case that just isn't met today by our existing transports.

[ghost]

I have not enough machines to test the performance of KCP protocol in Yggdrasil. So this PR is closed.