OAK-1249: Fine-grained locking in SegmentMK commits

Stick with a global lock for local commits for now until the merge feature gets optimized
Reduce the amount of synchronization in SegmentNodeState and Record

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1547081 13f79535-47bb-0310-9956-ffa450edef68