TAJO-1191: Change DateDatum timezone to UTC. (Jaewoong Jung via hyunsik)

Closes #278
TAJO-1194: 'INSERT OVERWRITE .. SELECT' does not remove existing data when result is empty. (jaehwa)

Closes #254
TAJO-1194: 'INSERT OVERWRITE .. SELECT' does not remove existing data when result is empty. (missing code)
Merge branch 'master' of http://git-wip-us.apache.org/repos/asf/tajo into index_support