Row set-based scan framework

Next step in the ongoing "Result Set Loader" saga. Merge the enhanced scan operator framework Into master. Includes:

• Projection add-ons for the "columns" column and file metadata columns.
• Mechanisms to orchestrate the result set loader, projection framework, scan node and reader code.
• Extensive unit tests.

Given the number of test files to review, pushed the following to the next PR:

• Extension to the "Easy" reader framework to handle the new structure.