Revert to Cohttp #279
Description
This is a warning to all users that we intend to revert back to Cohttp to Httpaf. The only reason for switching away from cohttp were:
- Cohttp's poor performance.
- Cohttp's dependence on conduit.
Since both of these issues are being resolved (mirage/ocaml-cohttp#819, mirage/ocaml-cohttp#821) there's no longer any incentive to use httpaf.