http2: only schedule write when necessary

Introduce an `Http2Scope` class that, when it goes out of scope,
checks whether a write to the network is desired by nghttp2.
If that is the case, schedule a write using `SetImmediate()`
rather than a custom per-session libuv handle.

PR-URL: https://github.com/nodejs/node/pull/17183


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