Without this change you need to do:

```
User user;
final SecurityContext oldContext = ACL.impersonate(a);
try {
    user = User.current();
} finally {
    SecurityContextHolder.setContext(oldContext);
}
```

Which seems incredibly wasteful.

Or else you need to replicate the logic in User.current() which makes that code hard to change.

This reverts commit fe1a1487.
It's been about a month, so I think it has served its purpose.

Allowing some methods to be overridden, with the caveat that the overrides will not be called from CoreWrapperStep.

[FIXED JENKINS-20989] PeepholePermalink RunListenerImpl oncompleted should be triggered before downstream builds are triggered

 - queueLengthSnapshot never used.

Ensure that listeners do not get notified when a computer is killed from a thread with the Queue lock

- We don't know what those listeners will be doing
- I'm not completely happy with having to do it this way, there are some theoretical paths where we end
  up re-acquiring the Queue lock in Computer.kill() but they should not be the normal path

IIUC, the expectation here is that Queue.Executable will instantiate AsynchronousException as a handle, start something asynchronously (say send a JMS message) with a callback, and the callback will tickle AsynchronousException.

So AsynchronousException might complete before it gets its executor set. This code change ensures that that case gets handled correctly.