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.
TAJO-868: TestDateTimeFunctions unit test is occasionally failed.

Closes #34
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-839
TAJO-839: If all tables participate in the BROADCAST JOIN, there is some missing data.
Remove unused codes.