Add Eio server benchmark

[Sudha247]

Adds a eio_server benchmark that I wrote for measuring the performance of cohttp-eio, similar to the async_server and lwt_unix_server_new benchmarks. Results obtained on a server machine (Intel Xeon Gold 5120) are below:

eio_server

141829 requests in 5.00s, 283.23MB read
Requests/sec:  28361.57
Transfer/sec:     56.64MB

async_server

105063 requests in 5.00s, 212.22MB read
Requests/sec:  21011.74
Transfer/sec:     42.44MB

lwt_unix_server_new

166961 requests in 5.00s, 333.42MB read
Requests/sec:  33393.19
Transfer/sec:     66.69MB

[talex5]

Thanks! This will be useful for anyone wanting to optimise it.

Note: ocaml-ci is failing with No solution found for any supported platform. This is because async requires OCaml < 5.1 and Eio requires OCaml >= 5.1. It is possible to work around this by putting an explicit constraint of the version of OCaml in the opam files (e.g.
if ocaml-ci sees that cohttp-eio requires OCaml 5.1 then it won't try to include cohttp-eio in the set for older versions).

[Sudha247]

Thank you, I'll take a stab at setting the constraints right.

[mseri]

I think it should be enough to add cohttp-eio and eio to the dependencies of cohttp-bench in the dune-project file and regenerate https://github.com/mirage/ocaml-cohttp/blob/master/cohttp-bench.opam

[mseri]

The remaining failures are CI errors related to homebrew and apt-get. I am merging