In SPM-4106 we want to split methods and data members of the DocumentSource class hierarchy and create a parallel class hierarchy responsible solely for query execution.

Since we don't want to do that in one commit, we will be splitting several classes per commit. In order to do that, we may create temporary wrapper classes for the not-yet-split classes that will refer to the original class.

At the end of the project we want to remove such wrapper classes and make sure that the new class hierarchy is loosely coupled with the DocumentSource class hierarchy, which will be responsible for parsing, validation, optimization, but not execution.

As a part of this ticket, we also want to ensure that the DocumentSource base class does not contain methods and data members responsible for query execution.