@Override
  public int unlockDatabase(Hive hiveDB, UnlockDatabaseDesc unlockDb) throws HiveException {
    super.unlockDatabase(hiveDB, unlockDb);
    throw new UnsupportedOperationException();
  }
Start the heartbeater threadpool and return the task.
@param initialDelay time to delay before first execution, in milliseconds
@return heartbeater
ql/src/java/org/apache/hadoop/hive/ql/lockmgr/DbTxnManager.java