src: use unrefed async for GC tracking

Do not let an internal handle keep the event loop alive.

PR-URL: https://github.com/nodejs/node/pull/16758
Fixes: https://github.com/node/issues/16210


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>