split cohttp-resto and rpc-http in a unix and js component

This MR split cohttp-resto in a unix and js client

To compile cohttp-lwt-jsoo and its dependency js_of_ocaml-lwt must be added to the opam-repository.

I've also splitted tezos-rpc-http in two new libraries tezos-rpc-http-client, tezos-rpc-http-server.

This work paves the way for a unix free tezos library