src: use unrefed async for GC tracking

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

I think this patch is okay since GC isn’t deterministic anyway.

Fixes: https://github.com/nodejs/node/issues/16210

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src

@trott @refack @jasnell