* Adding a write schema to columnar's merge operations.
* Added unit test checking min/max when columns are empty.
* CR comment
* Rename to value_type_to_column_type
Introduced asymetric OptionalCodec / SerializableOptionalCodec
Removed cardinality from the columnar sstable.
Added DynamicColumn
Reorganized all files
Change DenseCodec serialization logic.
Renamed methods to rank/select
Moved versioning footer to the columnar level
