OAK-690: Enforce and clarify Root contract wrt. invalid Tree instances
Initial implementation of enforcement. Currently disabled until all follow up issues this is causing are fixed. Use -DOAK-690=true to enable

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1455173 13f79535-47bb-0310-9956-ffa450edef68