KillQuery does not work query occasionally.

There are two types of killing queries. First is that QueryMaster cancel QueryUnitAttempt before QueryUnitAttempt be passed to TajoWorker. Second is that TajoWorker cancel running QueryUnitAttempt.

If QueryMaster cancel QueryUnitAttempts, it will be stopped. But unfortunately, at nearly the same time, a few QueryUnitAttempts can be passed to TajoWorker. And these never will be stopped because it can't find valid state for state machines.