http2: move uv_prepare handle to `Http2Session`

As far as I understand it, `Nghttp2Session` should not be concerned
about how its consumers handle I/O.

PR-URL: https://github.com/nodejs/node/pull/16461


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Franziska Hinkelmann <franziska.hinkelmann@gmail.com>