http2: keep session objects alive during Http2Scope

Keep a local handle as a reference to the JS `Http2Session`
object so that it will not be garbage collected
when inside an `Http2Scope`, because the presence of the
latter usually indicates that further actions on
the session object are expected.

Strictly speaking, storing the `session_handle_` as a
property on the scope object is not necessary, but
this is not very costly and makes the code more
obviously correct.

Fixes: https://github.com/nodejs/node/issues/17840

PR-URL: https://github.com/nodejs/node/pull/17863
Fixes: https://github.com/nodejs/node/issues/17840


Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>