TAJO-846: Clean up the task history in woker. (jinho)

Closes #20
TAJO-793: CLI should be able to exit when single query is failed. (Hyoungjun Kim via jinho)

Closes #25
TAJO-854: Supports INSERT INTO with UNION. (Hyoungjun Kim via jihoon)
TAJO-849: Add Parquet storage to HCatalogStore. (jaehwa)
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-774_2
TAJO-762: Implements current date/time function (Hyoungjun Kim via hyunsik)

Closes #16
TAJO-863: Column order mismatched in the JOIN query with asterisk selection. (Hyoungjun Kim via hyunsik)

Closes #31
TAJO-843: implements COALESCE for BOOLEAN, DATE, TIME, TIMESTAMP. (Hyoungjun Kim via hyunsik)

Closes #26
TAJO-860: Implements TRUNCATE table. (Hyoungjun Kim via hyunsik)
TAJO-860: Implements TRUNCATE table. (missed github closes)

Closes #29, #24.
Added more unit tests.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-774_2

Conflicts:
	tajo-core/src/main/java/org/apache/tajo/engine/planner/LogicalPlan.java
TAJO-868: TestDateTimeFunctions unit test is occasionally failed.

Closes #34
fixed some unit tests.
Fixed the bug with groupby.
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-774_2
Fixed all groupby bugs and added more unit tests.