OAK-2144: Intermittent Node not found at given revision with DocumentNodeStore

Detect when _lastRevs are ambiguous and use readRevision instead.
Enable tests.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1630156 13f79535-47bb-0310-9956-ffa450edef68
OAK-1941 - code cleanup

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1630166 13f79535-47bb-0310-9956-ffa450edef68