OAK-1822: NodeDocument _modified may go back in time

Additional test showing the diff issue when _modified is set back in time

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1594888 13f79535-47bb-0310-9956-ffa450edef68
OAK-1820: Set _lastRev on insert when new node is also commit root

Add missing DocumentNodeStore.dispose() at the end of the test.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1594889 13f79535-47bb-0310-9956-ffa450edef68