orca: fix race when calling listeners coincides with updating the run goroutine

[dfawley]

Blocking on p.stopped while holding p.mu can race if the run goroutine had just received a report and is trying to acquire p.mu in order to call the listeners.

Waiting for the run goroutine to exit before starting a new one is not necessary. This change will start a new one without waiting, and only wait during producer shutdown (which is also not strictly necessary, but is a good idea to ensure all resources are cleaned up).

RELEASE NOTES: none