async_hooks: minor cleanup and improvements

Cleanup some code and make the emit hooks very slightly faster.

PR-URL: https://github.com/nodejs/node/pull/27034


Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>