Added a latency probe to Entry Processor test

+ StringUtils refactored
Do we have a method which generates all possible characters instead of alphanumerics? Asking this, because the string generation strategy in this test https://github.com/hazelcast/performancetop5/blob/master/item1/src/main/java/com/hazelcast/performancetop5/item1/DistributedMapPutSetBenchmark.java showed a bug recently which related to some UTF encoding problems.