OAK-781: Clarify / fix effects of MISSING_NODE as base state of NodeBuilder
rework MemoryNodeBuilder:
- promote MutableNodeState to top level package private class
- separate state tracking and hierarchy tracking between MemoryNodeBuilder and MutableNodeState, respectively
- rename NodeBuilder methods to match those of NodeState

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1471355 13f79535-47bb-0310-9956-ffa450edef68
missing @Override

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1471356 13f79535-47bb-0310-9956-ffa450edef68
correctly sort modifiers

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1471357 13f79535-47bb-0310-9956-ffa450edef68