OAK-1586 - Implement checkpoint support in DocumentNodeStore

Adding a separate collection named 'settings' which stores the checkpoint data. It can be later used to store other state information related to DocumentNodeStore

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1581297 13f79535-47bb-0310-9956-ffa450edef68