split lightning's I_S.columns query into small ones to avoid slow query

[lance6716]

Enhancement

currently

tidb/pkg/lightning/backend/tidb/tidb.go

Lines 193 to 198 in aa83e4e

	rows, e := tx.Query(`
	SELECT table_name, column_name, column_type, generation_expression, extra
	FROM information_schema.columns
	WHERE table_schema = ?
	ORDER BY table_name, ordinal_position;
	`, schemaName)

will query for the whole database. We can only query needed tables and use proper batch size