HashAggCodeGen: Modify code generation for hash agg. Use workspace vectors.

Conflicts:
	exec/java-exec/src/main/java/org/apache/drill/exec/physical/impl/aggregate/AggBatch.java
HACodeGenMaster: Hack AggBatch.java to use a mappingset with "workspace" recordbatch.
Intermediate commit for aggr branch changes.  Also merge remote-tracking branch 'jni_aggr/HACodeGenMaster' into aggr_branch.
Cherry-pick changes from jni_aggr/HACodeGenMaster
Cherry-pick more changes from jni_aggr/HACodeGenMaster.
Fix some issues and intermediate commit for hash aggregation.