TAJO-829: Same constants in groupby clause may cause planning error.
TAJO-801: Multiple distinct should be supported. (Hyoungjun Kim via hyunsik)
TAJO-811: add simple fifo scheduler support. (jinho)
TAJO-833
TAJO-821: IllegalStateException occurs when a NettyClientBase object is created within single thread. (hyoungjunkim via jinho)
TAJO-833: NPE occurs when using the column as a alias name in the multiple DISTINCT. (Hyoungjun Kim via hyunsik)

Closes #11
TAJO-832: NPE occurs when Exception's message is null in Task. (Hyoungjun Kim via hyunsik)
TAJO-827: SUM() overflow in the case of INT4. (Hyoungjun Kim via hyunsik)
TAJO-808: Fix pre-commit build failure. (jinho)
TAJO-819: KillQuery does not work for running query on TajoWorker. (jaehwa)
TAJO-825: Datetime type refactoring. (Hyoungjun Kim via jihoon)
TAJO-825: Datetime type refactoring. (fixed missing changes)