OAK-2162: oak-it-mk: add ClientFixture

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1629163 13f79535-47bb-0310-9956-ffa450edef68
OAK-532: Inconsistent journal with concurrent updates
Improved error message

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1629168 13f79535-47bb-0310-9956-ffa450edef68