release savepoint behavior is inconsistent with mysql

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

/* test */ select version();
/* test */ drop table if exists t;
/* test */ create table t (k int primary key, v varchar(20));
/* test */ begin;
/* test */ insert into t values (1, 'a');
/* test */ savepoint p1;
/* test */ insert into t values (2, 'b');
/* test */ savepoint p2;
/* test */ select * from t;
/* test */ release savepoint p1;
/* test */ select * from t;
/* test */ rollback to p2; -- tidb: OK; mysql: SAVEPOINT p2 does not exist;
/* test */ select * from t;
/* test */ rollback;

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

master: 2c3f717