ENG-4096: optimize query only with GROUP BY on indexed columns (no WHERE, ORDER BY) to use IndexScan instead of SeqScan; moreover, apply SerialAgg on this IndexScan if it's unique.
ENG-4096: add test cases; use ParsedColInfo not groupbyExprs in ParsedSelectStmt to find the right index.
ENG-4096: remove unique check and change cooresponding test cases
ENG-4096: remove unnecessary unique index in test ddl; only apply this optimization when needed.
Merge branch 'master' into ENG-4096
ENG-4096: changes based on Paul's review.
Merge branch 'master' into ENG-4096

Conflicts:
	tests/frontend/org/voltdb/planner/TestPlansOrderBy.java
Add first step to implement where clause for re-aggregation plan node
Merge branch 'master' of github.com:VoltDB/voltdb into ENG-5177-mv-partitiion-where
Merge remote-tracking branch 'origin/release-3.7' into ENG-4096

Conflicts:
	src/frontend/org/voltdb/planner/PlanAssembler.java
	tests/frontend/org/voltdb/planner/TestPlansGroupBy.java
ENG-4744, ENG-4096: do NOT re-use reAggNode for multiple plans; solve conflicts between these two tickets.
Add support for re-aggregation query with where clause
ENG-4906: add more test case and other minor changes.
Merge branch 'release-3.7' into ENG-5177-mv-partitiion-where
Add planner test case for mv fixed query with where clause
Merge branch 'release-3.7' of github.com:VoltDB/voltdb into ENG-5177-mv-partitiion-where
Ignore the DEFAULT NULL in ddl
Add test case for defaul null
refix according to Paul's suggestion and add expected test case for future use
Merge branch 'release-3.7' of github.com:VoltDB/voltdb into ENG-5177-mv-partitiion-where