@Override
      public long currentTimeMillis() {
        return System.currentTimeMillis();
      }
@param messagesPerBatch
         The number of messages to add to the BlockingEnvelopeMap before
         sleeping.
@param threadCount
         How many threads to run.
@param brokerSleepMs
         How long each thread should sleep between batch writes.
samza-test/src/main/java/org/apache/samza/system/mock/MockSystemConsumer.java