SAVEPOINT support

[yoshinorim]

SAVEPOINT is needed for https://bugs.mysql.com/bug.php?id=71017. For bug71017, SAVEPOINT for read only transaction is good enough. It is needed to define behavior if there is any updates.

• Supporting real SAVEPOINT (rolling back to the savepoint if executing ROLLBACK TO SAVEPOINT).
• Returning errors if there is any modification after or at executing SAVEPOINT.

[yoshinorim]

First diff was committed -- https://reviews.facebook.net/D44223 . This is for read only transactions (mysqldump) only, and there is no error handling on modifications.