Rewrite beast C++ test server

[sehe]

This rewrite was prompted by the issue opened on the beast repository: https://github.com/boostorg/beast/issues/2883

You can see the relative timings (there are several choices) there. Suffice it to say that the fast config is 10x faster than the original.

From the commit message:

This happens to use C++20 coroutines because I'm lazy. It can equivalently be written without.

I reworded measure.sh to use cmake instead of zig build. Again, I'm lazy and don't wish to learn zig build today.

The Nix flake had dependency issues

• for one, the beast archive references in cpp/build.zig.zon is gone;
• secondly the upstream zig flake does not have zig 0.12.0 ¯\(ツ)/¯ Luckily that's in Nixpkgs, so I switched to nixpkgs-unstable.
• I also added a liburing dependency to the devShell. This was only temporary to test with uring, but it didn't produce better results