Fix data race on Checksumming field in TGroupSessions (#26096)

[serbel324]

Changelog entry

Fixes #26059

Changelog category

• Bugfix

Description for reviewers

Checksumming field may be accessed from different threads, so we make it atomic.

[github-actions]

⚪ 2025-10-02 10:19:45 UTC Pre-commit check linux-x86_64-relwithdebinfo for c6db312 has started.
⚪ 2025-10-02 10:19:49 UTC Artifacts will be uploaded here
⚪ 2025-10-02 10:23:20 UTC ya make is running...
🟡 2025-10-02 11:48:39 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
38417	35618	0	6	2759	34

⚪ 2025-10-02 11:52:05 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-10-02 12:05:27 UTC Tests successful.

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
489 (only retried tests)	367	0	0	108	14

🟢 2025-10-02 12:05:36 UTC Build successful.
🟢 2025-10-02 12:05:52 UTC ydbd size 2.3 GiB changed* by +10.4 KiB, which is < 100.0 KiB vs stable-25-3-1: OK

ydbd size dash	stable-25-3-1: cd13161	merge: c6db312	diff	diff %
ydbd size	2 431 169 384 Bytes	2 431 180 024 Bytes	+10.4 KiB	+0.000%
ydbd stripped size	520 829 416 Bytes	520 830 184 Bytes	+768 Bytes	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

⚪ 2025-10-02 10:20:28 UTC Pre-commit check linux-x86_64-release-asan for c6db312 has started.
⚪ 2025-10-02 10:20:32 UTC Artifacts will be uploaded here
⚪ 2025-10-02 10:24:13 UTC ya make is running...
🟡 2025-10-02 12:27:54 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
15761	15309	0	167	254	31

🟢 2025-10-02 12:29:14 UTC Build successful.
🟢 2025-10-02 12:29:37 UTC ydbd size 3.8 GiB changed* by +21.3 KiB, which is < 100.0 KiB vs stable-25-3-1: OK

ydbd size dash	stable-25-3-1: cd13161	merge: c6db312	diff	diff %
ydbd size	4 044 978 664 Bytes	4 045 000 448 Bytes	+21.3 KiB	+0.001%
ydbd stripped size	1 506 892 800 Bytes	1 506 898 432 Bytes	+5.5 KiB	+0.000%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}

[github-actions]

🟢 2025-10-02 10:20:56 UTC The validation of the Pull Request description is successful.