OAK-1586 - Implement checkpoint support in DocumentNodeStore

Adding support for collecting expired checkpoints in create path. This should help in mitigating the case where checkpoints are frequently created (OAK-1647) .

 This should fare better compared to current impl where checkpoint were only deleted when oldest checkpoint is fetched

Also added an ignored testcase for OAK-1648

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1583251 13f79535-47bb-0310-9956-ffa450edef68