OAK-1486: BackgroundObserverTest occasionally failing
- Fix test to correctly apply memory barrier wrt. the returned assertions
- Revert wrong changes in BackgroundObserver from r1573571

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1573682 13f79535-47bb-0310-9956-ffa450edef68