WIP: Support offsets and limited win functions

Conflicts:
	exec/java-exec/src/main/java/org/apache/drill/exec/expr/fn/MethodGrabbingVisitor.java
	exec/java-exec/src/main/java/org/apache/drill/exec/planner/logical/DrillAggregateRel.java
	exec/java-exec/src/main/java/org/apache/drill/exec/planner/logical/DrillWindowRel.java
	exec/java-exec/src/main/java/org/apache/drill/exec/planner/physical/WindowPrel.java