DRILL-7024: Refactor ColumnWriter to simplify type-conversion shim
DRILL-7006 added a type conversion "shim" within the row set framework. Basically, we insert a "shim" column writer that takes data in one form (String, say), and does reader-specific conversions to a target format (INT, say).
The code works fine, but the shim class ends up needing to override a bunch of methods which it then passes along to the base writer. This PR refactors the code so that the conversion shim is simpler.
closes #1633
