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

Simplify MutableNodeState.snapshot() by moving the related logic to the ModifedNodeState constructor.
No more need for ModifiedNodeState.collapse() as we take care of the unwrapping already in MutableNodeState.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1477671 13f79535-47bb-0310-9956-ffa450edef68