Improve DistributedLogServerTestCase

See https://builds.apache.org/job/distributedlog-precommit-pullrequest/38/com.twitter$distributedlog-service/testReport/

Currently, the port is hard-code:

    @Before
    public void setup() throws Exception {
        dlServer = createDistributedLogServer(7001);
        dlClient = createDistributedLogClient("test");
    }

Therefore "java.net.BindException: Address already in use" will be seen under a not-cleaned or shared env. we should introduce a fall-back mechanism