Compact value columns aren't being migrated properly in 3.0

To quote Tyler:

2.1 and 3.0 currently have different behavior around default compact value columns. When you create a table like this:

CREATE TABLE foo (
    k int,
    c int,
    PRIMARY KEY (k, c)
) WITH COMPACT STORAGE;

2.1 will add a compact_value column to system.schema_columns with an empty column_name and a BytesType validator.

In 3.0, we instead add a regular column with the default compact value name (value) and an EmptyType validator.

The logic in 3.0 depends on having an EmptyType column (see CompactTables.hasEmptyCompactValue()) but current trunk doesn't migrate the column. LegacySchemaMigrator.addDefinitionForUpgrade() almost does what we want, but doesn't add the EmptyType column because it sees the existing compact_value column.