OAK-1586 - Implement checkpoint support in DocumentNodeStore

Initial implementation which stores the checkpoint data as part of NODES collection

-- Using Clock for determining current time to simplify testing
-- Custom Clock can be specified via Builder

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1580769 13f79535-47bb-0310-9956-ffa450edef68