OAK-781: Clarify / fix effects of MISSING_NODE as base state of NodeBuilder

No more need to get a NodeState snapshot for the exists() call in canRead(TreeImpl)

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1476101 13f79535-47bb-0310-9956-ffa450edef68