NNThroughputBenchmark should support specifying the base directory for multi-client test

Currently, NNThroughputBenchmark does not support specifying the base directory, therefore does not support multiple clients performing stress testing at the same time. However, for high-performance namenode machine, only one client submitting stress test can not make the namenode rpc access reach the bottleneck. Therefore, multiple clients are required for parallel testing to make the namenode pressure reach the level of the large-scale production cluster.

So I specify the base directory through the -baseDirName parameter to support multiple clients submitting stress tests at the same time.