During development of the $merge stage we realized that the existing concept of DocumentSource::MergingLogic now had the potential to be confused with something specific to the $merge stage or with similarly named types we're introducing, MergeStrategyDescriptor and MergeStrategy.

This ticket proposes renaming 'MergingLogic' and 'mergingLogic()' to be 'DistributedPlanLogic' and 'distributedPlanLogic()' to make the distinction more clear.