OAK-1586: Implement checkpoint support in DocumentNodeStore

The virtual clock idea is useful enough to be moved from the test case to Clock
Also clarified the ACCURATE clock functionality and added toString() methods

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1581203 13f79535-47bb-0310-9956-ffa450edef68
OAK-1594: Build fails on Java 8

Ignore ordering when comparing sets of privileges

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1581204 13f79535-47bb-0310-9956-ffa450edef68