TAJO-816: NULL delimiter doesn't apply with HCatalogStore. (jaehwa)
TAJO-829: Same constants in groupby clause may cause planning error.
TAJO-801: Multiple distinct should be supported. (Hyoungjun Kim via hyunsik)
TAJO-811: add simple fifo scheduler support. (jinho)
Merge branch 'master' of https://git-wip-us.apache.org/repos/asf/tajo into TAJO-774

Conflicts:
	tajo-core/src/main/java/org/apache/tajo/engine/planner/BasicLogicalPlanVisitor.java
	tajo-core/src/main/java/org/apache/tajo/engine/planner/ExplainLogicalPlanVisitor.java
	tajo-core/src/main/java/org/apache/tajo/engine/planner/LogicalPlanVisitor.java
	tajo-core/src/main/java/org/apache/tajo/engine/planner/PhysicalPlannerImpl.java
improved physical planner.