@After
  public void tearDown() throws Exception {
    TestTxnDbUtil.cleanDb(conf);
  }
counter = 0;
Thread1 counter=1, lock, wait 3s, check counter(should be 2), counter=3, unlock
Thread2 counter=2, lock (and block), inc counter, should be 4
ql/src/test/org/apache/hadoop/hive/metastore/txn/TestTxnHandler.java