async_hooks: improve AsyncResource performance

Accessing symbols is generally quite expensive and so is emitInit,
only do both when actually required.

PR-URL: https://github.com/nodejs/node/pull/27032


Reviewed-By: Gus Caplan <me@gus.host>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: James M Snell <jasnell@gmail.com>