legacy_schema_migrator: Migrate columns like Cassandra
legacy_schema_migrator: Migrate columns like Cassandra

This fixes generation of synthetic columns for static compact tables.
Current code always generates synthetic clustering column with utf8
type and synthetic regular column with bytes type (in schema_builder).
That's fine when creating a new CQL table, but not when migrating
existing tables created via thrift API.

Fixes #2584.

This also migrates empty compact value columns like Cassandra
does. Such columns are present in compact tables without regular
columns, e.g.:

  create table test (k int, ck int, primary key (k, ck)) with compact storage;

They should be migrated to a synthetic regular column with
empty_type type and a non-empty name.

scylladb/scylla,duarten/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,duarten/scylla,duarten/scylla