txn: the tidb_low_resolution variable is used on select for update statement

[cfzjywxk]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

create table t(a int key);

set tidb_low_resolution_tso = 'ON';
begin;
select * from t where a = 1 for update; // or select * from t for update

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

The select for update should report an error that when tidb_low_resolution_tso is set, the statement could not be executed.

3. What did you see instead (Required)

The select for update statement executes successfully.

4. What is your TiDB version? (Required)

Nightly with ca7ba14da87abd060922b32c893b121c4c3d6373 commit hash