Replace raw pointer buffer access with static `Column` `for_each`/`transform` methods or `ColumnDataIterator` where possible

[alexowens90]

Known locations:

• [x] copy_frame_data_to_buffer
• [ ] LocalVersionedEngine::get_descriptor x2
• [ ] read_frame.cpp multiple locations
• [ ] aggregator_set_data
• [x] AggregationClause::process x2 closed by #1450
• [x] aggregation.cpp all finalize methods closed by #1450