src: minor updates to FastHrtime

- Don’t use 12 as a magic number for the buffer size
- Mark the object as weak (which is conceptually the right thing to do,
  even if there is little practical impact)
- Keep a reference to the `ArrayBuffer` in question for memory tracking

PR-URL: https://github.com/nodejs/node/pull/33851


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>