http2: track async state for sending

Sending pending data may involve running arbitrary JavaScript code.
Therefore, it should involve a callback scope.

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>