OAK-1600: Slow ConcurrentReadIT

Remove TODO. Slow down is caused by increased cost of AbstractTree.getChildNames() and the fact that the test accesses a session with multiple threads.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1582647 13f79535-47bb-0310-9956-ffa450edef68