http2: use session not socket timeout, tests

Change default timeout to be tracked on the session instead
of the socket, as nghttp2 manages the socket and we would
need to maintain two sets of timeouts for similar purpose.
Also fixes session setTimeout to work as it wasn't getting
_unrefActive correctly (was called on the handle).

Fixes: https://github.com/nodejs/node/issues/15158
PR-URL: https://github.com/nodejs/node/pull/15188


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>