OAK-641: Improved benchmark tooling - remove the lower limit for number of iterations (it is currently a problem for the XmlImportTest with MongoMK), and change the warmup so it is seconds instead of iterations


git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1527910 13f79535-47bb-0310-9956-ffa450edef68