http2: make sessions garbage-collectible

Use weak handles to track the lifetime of an `Http2Session` instance.

Refs: https://github.com/nodejs/http2/issues/140
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>