Current implementation of server-reason-react depends on Lwt

[davesnx]

After the RSC work, big dependencies are introduced in the project, which means that users of server-reason-react might need to install it regardless if they don't use it; and in the worst-case scenario it won't be compatible with their set of dependencies.

If @tjdevries wants to use it with Eio for example, he would need a new package server-reason-react-eio (?) with a new implementation.

Similarly if someone doesn't want to use yojson and want to use jsont they won't be able to.