WIP:Joezhong fix 320

[Beastjoe]

Related Issue #320

Sorry for the delay. I tried to solve the case when scramble tables are involved in set operation. Currently, no AsyncAggExecutionNode is created if scramble tables are in set operation. The reason is that VerdictDB will select and group by verditdbtier column. However, in set operation, the number of selected columns should be the same. For instance,
select count(something) from (select something from scramble_table UNION select something from normal_table)
If we try to use AsyncAggExecutionNode here, the query created by the individual AggExecutionNode will become like
select something, verdictdbtier from scramble_table where verdictdbblock>=0 UNION select something from normal_table

[codecov-io]

Codecov Report

Merging #323 into master will increase coverage by 0.33%.
The diff coverage is 81.04%.

@@            Coverage Diff             @@
##           master     #323      +/-   ##
==========================================
+ Coverage   70.62%   70.94%   +0.33%     
==========================================
  Files         168      168              
  Lines       11492    11696     +204     
  Branches     1874     1935      +61     
==========================================
+ Hits         8115     8297     +182     
- Misses       2891     2904      +13     
- Partials      486      495       +9

Impacted Files	Coverage Δ
.../main/java/org/verdictdb/sqlwriter/QueryToSql.java	64% <0%> (-4%)	⬇️
...verdictdb/core/querying/ola/InMemoryAggregate.java	83.46% <100%> (+0.12%)	⬆️
...ictdb/core/querying/QueryNodeWithPlaceHolders.java	88.28% <100%> (ø)	⬆️
...java/org/verdictdb/sqlwriter/SelectQueryToSql.java	83.16% <100%> (+0.45%)	⬆️
...org/verdictdb/sqlreader/ScrambleTableReplacer.java	78% <33.34%> (-4.22%)	⬇️
...verdictdb/core/sqlobject/SetOperationRelation.java	58.63% <52.64%> (-3.28%)	⬇️
...va/org/verdictdb/coordinator/ExecutionContext.java	85.1% <66.67%> (+0.09%)	⬆️
...main/java/org/verdictdb/sqlreader/RelationGen.java	89.31% <66.67%> (-2.5%)	⬇️
.../org/verdictdb/sqlreader/RelationStandardizer.java	79.88% <85.72%> (+0.4%)	⬆️
...tdb/core/querying/ola/AsyncQueryExecutionPlan.java	84.27% <88.6%> (-0.54%)	⬇️
... and 10 more

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[Beastjoe]

@pyongjoo
I think now set operation is supported for async aggregation. When scramble tables have different probability distribution, VerdictDB will not create asyncAggExecutionNode.