OAK-1586 - Implement checkpoint support in DocumentNodeStore

Updating the testcase to account for changes in initial cache state.

Testcase logic was bit fragile wrt initial cache state. Making it more robus

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1580776 13f79535-47bb-0310-9956-ffa450edef68