Add mysqli_rollback_to

Description

In PHP 5.5 mysqli::savepoint and mysqli::release_savepoint were added, but not rollback_to. Maybe there is some database engine where savepoints have multiple purposes, but in mariadb the only use of savepoints is to call query ROLLBACK TO name manually.

For completion: existing rollback with $name is doing nothing (only adding comment) at least in mariadb/mysql.

Method signature

class mysqli { /* ... */
  public function rollback_to(string $name) : bool;
}

Sample test sequence

mysqli_query($link, 'DROP TABLE IF EXISTS test');
mysqli_query($link, 'CREATE TABLE test(id INT) ENGINE = InnoDB');

mysqli_begin_transaction($link);
mysqli_query($link, 'INSERT INTO test(id) VALUES (1)');
mysqli_savepoint($link, 'foo');
mysqli_query($link, 'INSERT INTO test(id) VALUES (2)');
mysqli_rollback_to($link, 'foo');
mysqli_commit($link);
$res = mysqli_query($link, "SELECT * FROM test");
var_dump($res->num_rows); // int(1)

まず、既存の mysqli::savepoint、mysqli::release_savepoint、mysqli::rollback のエントリーポイントと、それらに関連するテストを追跡します。サンプルトランザクションシーケンスを動作確認に使用してください：savepoint までロールバックすると、commit 後も最初の insert を保持したまま、2番目の insert が削除されるはずです。