Convert http3-server to use Tokio instead of mio

[agrover]

Could be cool, and possibly interesting to see how Neqo worked with an async event model. It already kind of does so this might actually be a good fit.

[xpepermint]

Actually, it would be great to support arbitrary async runtime.

[larseggert]

CC @mxinden

[mxinden]

I am not sure what http3-server refers to nowadays. #1581 and #1612 move neqo-server and neqo-client to tokio.

While I believe asynchronicity is a helpful tool, I do very much agree with the argument that it adds complexity and that it is infectious to a codebase (e.g. read "what color is your function"). Thus I suggest driving I/O in an asynchronous fashion, but keeping the core of neqo (i.e. neqo-common, neqo-transport, neqo-http3, neqo-crypto) synchronous.

With the two pull requests and the above argumentation, I believe this can be closed. Agreed @larseggert?