async_wrap: schedule destroy hook as unref

Since the `DestroyAsyncIdsCallback` in Node.js can be scheduled as
a result of GC, that means that it can accidentally keep the event
loop open when it shouldn't.

Replace `SetImmediate` with the newly introduced `SetUnrefImmediate`
and in addition introduce RunBeforeExit callbacks, of which
`DestroyAsyncIdsCallback` is now the first. These callbacks will run
before the `beforeExit` event is emitted (which will now only be
emitted if the event loop is still not active).

PR-URL: https://github.com/nodejs/node/pull/18241
Fixes: https://github.com/nodejs/node/issues/18190


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>