OAK-1178: MutableTree#isNew: replace implementation by NodeBuilder#isNew
- Rebasing should correctly set the base state of the KernelNodeBuilder and the MongoNodeBuilder
- MongoNodeBuilder needs to calculate its base state instead of relying on that of the MemoryNodeBuilder backing it

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1541707 13f79535-47bb-0310-9956-ffa450edef68