http: allow _httpMessage to be GC'ed

Set `socket._httpMessage` to `null` before emitting the `'connect'` or
`'upgrade'` event.

PR-URL: https://github.com/nodejs/node/pull/18865


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>