OAK-798: Review / refactor TreeImpl and related classes
OAK-709: Consider moving permission evaluation to the node state level
- Introduce @NonNull variants of Root.getTree, Tree.getParent and Tree.getChild
- Implement @CheckForNull variants in term of @NonNull variants
- Clean up TreeImpl
- Rename Tree.isConnected to Tree.exists

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1477007 13f79535-47bb-0310-9956-ffa450edef68