OAK-1170: Inconsistent reads with concurrent benchmark tests
- make external changes only visible in background read, based on _lastRev available on root document
- remove MongoNodeStore.publishRevision()
- adjust some tests because the implementation doesn't try to make external changes visible anymore on a conflict
- enable test added for this issue

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1541915 13f79535-47bb-0310-9956-ffa450edef68