WIP: Occasional scaling error fix

[pyongjoo]

I placed an objective on ExecutableNodeRunner.run() for serialized executions. This seems to be helpful when multiple results are passed concurrently from individual aggregations.

[codecov-io]

Codecov Report

Merging #317 into master will decrease coverage by 0.05%. The diff coverage is 90.7%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
- Coverage   70.72%   70.68%   -0.04%     
==========================================
  Files         167      167              
  Lines       11208    11178      -30     
  Branches     1841     1832       -9     
==========================================
- Hits         7926     7900      -26     
- Misses       2823     2824       +1     
+ Partials      459      454       -5

Impacted Files	Coverage Δ
.../verdictdb/core/execplan/ExecutablePlanRunner.java	95% <ø> (ø)	:arrow_up:
...ava/org/verdictdb/metastore/ScrambleMetaStore.java	96.08% <ø> (ø)	:arrow_up:
...va/org/verdictdb/coordinator/ExecutionContext.java	87.25% <ø> (ø)	:arrow_up:
...erdictdb/core/querying/SelectAggExecutionNode.java	93.11% <100%> (-0.44%)	:arrow_down:
...verdictdb/core/querying/ola/InMemoryAggregate.java	87.69% <100%> (+2.79%)	:arrow_up:
.../java/org/verdictdb/core/querying/ola/AggMeta.java	91.67% <100%> (+0.08%)	:arrow_up:
.../verdictdb/core/execplan/ExecutableNodeRunner.java	80.26% <100%> (-0.16%)	:arrow_down:
...core/querying/ola/SelectAsyncAggExecutionNode.java	93.25% <73.34%> (+10.25%)	:arrow_up:
...rg/verdictdb/exception/VerdictDBDbmsException.java	50% <0%> (-50%)	:arrow_down:
...or/QueryResultAccuracyEstimatorFromDifference.java	90.37% <0%> (-2.4%)	:arrow_down:
... and 5 more

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