OAK-853: Many child nodes: Diffing causes many calls to the MicroKernel

Avoid storing ModifiedNodeStates in MemoryNodeStore; squeeze them instead to MemoryNodeStates

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1489884 13f79535-47bb-0310-9956-ffa450edef68