OAK-926: MongoMK: split documents when they are too large
- Move more node specific code from MongoMK to NodeDocument
- Centralize access to revisions and commit root in NodeDocument

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1514457 13f79535-47bb-0310-9956-ffa450edef68