OAK-1341 - DocumentNodeStore: Implement revision garbage collection (WIP)

Fix the modifiedTime calculation logic

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582657 13f79535-47bb-0310-9956-ffa450edef68
OAK-1341 - DocumentNodeStore: Implement revision garbage collection (WIP)

 Testing more code paths for deletion case

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582658 13f79535-47bb-0310-9956-ffa450edef68