OAK-1156 - Improve the document cache invalidation logic to selectivly invalidate doc

Changes as per review comments - Removing the TODO around lock handling as no locks are
required in this access

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1542184 13f79535-47bb-0310-9956-ffa450edef68