Refactor QueryProcessor::decomposePlanIntoTasks

[rfdavid]

As discussed with @andyfengHKU, cases like COPY_TO end up in the middle of the pipeline, example
ResultCollector} -> {FTableScan -> CopyTo} -> {FTableScan -> ResultCollector}
where only the middle pipeline needs to be executed in a single thread.

The additional code was added as a temporary solution inside this function:

if (op->getOperatorType() == PhysicalOperatorType::COPY_TO) {
  childTask->setSingleThreadedTask();
}