Splits and validate the cf parameter, containing an optional
comma-separated list of table names.
If any table is not found and a no_such_column_family
exception is thrown, wrap it in a `bad_param_exception`
so it will translate to `reply::status_type::bad_request`
rather than `reply::status_type::internal_server_error`.
With that, hide the split_cf function from api/api.hh
since it was used only from api/storage_service
and new use sites should use validate_tables instead.
Fixes#9754
Signed-off-by: Benny Halevy <bhalevy@scylladb.com>