OAK-1031: SegmentMK: Fewer segment lookups

Make SegmentNodeState extend Record and use the built-in segment tracking mechanism.
Clean up the PropertyState equality definition since now also Blob equality is better defined.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1526767 13f79535-47bb-0310-9956-ffa450edef68