OAK-1031: SegmentMK: Fewer segment lookups

Merge MapLeaf and MapBranch to the MapRecord base class so we can instantiate it without reading the record.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1528979 13f79535-47bb-0310-9956-ffa450edef68