store/tikv: fix a memory leak in the batchClient for the large transactions (#14031)

[sre-bot]

cherry-pick #14031 to release-3.0

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What problem does this PR solve?

Fix a memory leak.

What is changed and how it works?

Change xxx = xxx[:0] to some deep reset operation.

Check List

Tests

• Manual test (add detailed scripts or steps below)

Run some large transactions severals and observe the heap pprof.

mysql> truncate table sbtest2;                                                                                                                                                                        Query OK, 0 rows affected (1.15 sec)

mysql> insert into sbtest2 select * from sbtest1 limit 15000000;                                                                                                                                      Query OK, 15000000 rows affected (4 min 48.79 sec)
Records: 15000000  Duplicates: 0  Warnings: 0

mysql> truncate sbtest2;
Query OK, 0 rows affected (1.15 sec)

mysql> insert into sbtest2 select * from sbtest1 limit 10000000;
Query OK, 10000000 rows affected (2 min 59.24 sec)
Records: 10000000  Duplicates: 0  Warnings: 0

mysql> truncate sbtest2;
Query OK, 0 rows affected (1.04 sec)

mysql> insert into sbtest2 select * from sbtest1 limit 5000000;
Query OK, 5000000 rows affected (1 min 30.57 sec)
Records: 5000000  Duplicates: 0  Warnings: 0

Side effects

• Possible performance regression

Related changes

• Need to cherry-pick to the release branch

Release note

• Write release note for bug-fix or new feature.

[sre-bot]

/run-all-tests

[coocood]

LGTM

[jackysp]

LGTM

[jackysp]

/merge

[sre-bot]

Your auto merge job has been accepted, waiting for 14026, 13998

[sre-bot]

/run-all-tests