Run-away threads when using the JCR ObservationManager

In our application, we're seeing run-away threads when using the ObservationManager. Our application creates a new session for every HTTP request, and uses the ObservationManager to register some contextual data. When we invoke Workspace#getObservationManager, it appears that we create a new modeshape-event-dispatcher thread that is not disposed off when Session#logout is called.

Not that this is a very good demo, but I hope it gets the point across:

    @Test
    @FixFor("MODE-xxxx")
    public void shouldNotLeakObservers() throws Exception {
        final int oldCount = Thread.activeCount();
        for (int i = 0; i < 1000; i++) {
            final Session newSession = newSession();
            newSession.getWorkspace().getObservationManager();
            newSession.logout();
        }
        final int newCount = Thread.activeCount();

        assertEquals(oldCount, newCount);
        /* After running the test, these counts are ~1000 different */
    }

Digging into the code, I suspect (though haven't confirmed) that the ObservationManager's own listener is not removed:

https://github.com/ModeShape/modeshape/blob/master/modeshape-jcr/src/main/java/org/modeshape/jcr/JcrObservationManager.java#L174