http2: refactor ping + settings object lifetime management

Have clearer ownership relations between the `Http2Ping`,
`Http2Settings` and `Http2Session` objects.

Ping and Settings objects are now owned by the `Http2Session`
instance, and deleted along with it, so neither type of object
refers to the session after it is gone.
In the case of `Http2Ping`s, that deletion is slightly delayed,
so we explicitly reset its `session_` property.

Fixes: https://github.com/nodejs/node/issues/28088

PR-URL: https://github.com/nodejs/node/pull/28150


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Ujjwal Sharma <usharma1998@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>