TAJO-996: Sometimes, scheduleFetchesByEvenDistributedVolumes loses some FetchImpls.

Closes #109
TAJO-994: 'count(distinct x)' function counts first null value.

Closes #106
TAJO-999: SequenceFile key class need to be compatible. (jaehwa)

Closes #110
TAJO-928: Session variables should override query configs in TajoConf.

Closes #98