OAK-1941 - optimize cmodcount handling, add test case

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1642886 13f79535-47bb-0310-9956-ffa450edef68
OAK-1941 - remove special-handling of _collisionsModCount

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1642888 13f79535-47bb-0310-9956-ffa450edef68