TAJO-1261: Separate query and ddl execution codes from GlobalEngine.

Closes #312
TAJO-1250: RawFileAppender occasionally causes BufferOverflowException. (jinho)

Closes #303
TAJO-1262: Rename the prefix 'SubQuery' to 'Stage'.

Closes #314
TAJO-1268: tajo-client module should not use UserGroupInformation.