executor, expression: fix current_timestamp/now not consistent in one stmt like MySQL (#11342)

[cfzjywxk]

What problem does this PR solve?

cherry-pick for #11342.

MySQL will set time to “”time_cache“ structure before doing one command(COM_XXX)
later expresission calculation will use this timestamp as time value

TiDB logic different and default column calculation and now expr calculation will try to get timestamp twice.

more details see jira issue#4391
eg:
TiDB

CREATE TABLE `tt4` (
  `c1` timestamp DEFAULT CURRENT_TIMESTAMP,
  `c2` int(11) DEFAULT NULL,
  `c3` timestamp DEFAULT CURRENT_TIMESTAMP
) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin

insert into tt4 set c1 = current_timestamp, c2 = sleep(2);
mysql> select c1 = c3 from tt4;
+---------+
| c1 = c3 |
+---------+
|       0 |
+---------+
1 row in set (0.00 sec)

MySQL

insert into tt4 set c1 = current_timestamp, c2 = sleep(2);
select c1 = c3 from tt4;
+---------+
| c1 = c3 |
+---------+
|       1 |
+---------+
1 row in set (0.00 sec)

remove stmtCtx.sysTs unused variable
cache time value once in stmtCtx.nowTs
refactor nowTs usage, offer Unified function interface for get and reset nowTS. Generally, accessing variable directly is not a good way for further maintenance, refactor these usage code

What is changed and how it works?

Check List

Tests

• Unit test

Code changes

• Has exported function/method change
• Has exported variable/fields change
• Has interface methods change

Side effects

[cfzjywxk]

/run-all-tests

[cfzjywxk]

/run-all-tests

[cfzjywxk]

/run-all-tests

[zz-jason]

LGTM

[qw4990]

LGTM