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
TAJO-947: ColPartitionStoreExec can cause URISyntaxException due to special characters. (Mai Hai Thanh via hyunsik)
TAJO-995: HiveMetaStoreClient wrapper should retry the connection. (jinho)

Closes #108
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-895

Conflicts:
	tajo-core/src/main/java/org/apache/tajo/engine/planner/LogicalPlanPreprocessor.java
	tajo-core/src/main/java/org/apache/tajo/engine/planner/LogicalPlanner.java
	tajo-core/src/test/java/org/apache/tajo/cli/TestTajoCli.java