async_hooks: use scope for defaultTriggerAsyncId

Previously the getter would mutate the kDefaultTriggerAsncId value. This
refactor changes the setter to bind the current kDefaultTriggerAsncId to
a scope, such that the getter doesn't have to mutate its own value.

PR-URL: https://github.com/nodejs/node/pull/17273


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>