DataShard: prioritize cancellation processing for reads

[snaury]

Changelog entry

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Description for reviewers

When datashard has a lot of simple incoming read requests the latency may grow and read requests are eventually cancelled by latency sensitive clients. However, this cancellation had no chance of being processed in many cases, which caused a metastable state where datashard has a long request queue in the mailbox, processing cancelled requests. Use low-priority transactions when starting reads, which makes read cancellation higher priority.

Fixes #14936.

[github-actions]

🟢 2025-03-06 09:52:38 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-03-06 09:54:34 UTC Pre-commit check linux-x86_64-relwithdebinfo for 7741198 has started.
⚪ 2025-03-06 09:54:50 UTC Artifacts will be uploaded here
⚪ 2025-03-06 09:57:41 UTC ya make is running...
🟡 2025-03-06 10:49:33 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
19199	17853	0	3	1222	121

⚪ 2025-03-06 10:51:12 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-03-06 11:04:27 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
177 (only retried tests)	63	0	2	4	108

⚪ 2025-03-06 11:04:39 UTC ya make is running... (failed tests rerun, try 3)
🔴 2025-03-06 11:18:08 UTC Some tests failed, follow the links below.

Test history | Ya make output | Test bloat | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
156 (only retried tests)	49	0	2	0	105

🟢 2025-03-06 11:18:24 UTC Build successful.
🟢 2025-03-06 11:18:44 UTC ydbd size 2.1 GiB changed* by +48 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 7fc471f	merge: 7741198	diff	diff %
ydbd size	2 293 146 328 Bytes	2 293 146 376 Bytes	+48 Bytes	+0.000%
ydbd stripped size	480 458 432 Bytes	480 458 368 Bytes	-64 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]

⚪ 2025-03-06 09:54:51 UTC Pre-commit check linux-x86_64-release-asan for 7741198 has started.
⚪ 2025-03-06 09:55:06 UTC Artifacts will be uploaded here
⚪ 2025-03-06 09:57:47 UTC ya make is running...
🟡 2025-03-06 11:06:21 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
11735	11364	0	259	74	38

⚪ 2025-03-06 11:07:25 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-03-06 11:20:55 UTC ydbd size 3.7 GiB changed* by +82.8 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 89fbf3d	merge: 7741198	diff	diff %
ydbd size	3 993 538 432 Bytes	3 993 623 224 Bytes	+82.8 KiB	+0.002%
ydbd stripped size	1 388 716 392 Bytes	1 388 750 792 Bytes	+33.6 KiB	+0.002%

^{*please be aware that the difference is based on comparing your commit and the last completed build from the post-commit, check comparation}
⚫ 2025-03-06 11:20:56 UTC Check cancelled

[github-actions]

⚪ 2025-03-06 11:23:12 UTC Pre-commit check linux-x86_64-release-asan for e126fe7 has started.
⚪ 2025-03-06 11:23:26 UTC Artifacts will be uploaded here
⚪ 2025-03-06 11:26:23 UTC ya make is running...
🟡 2025-03-06 12:39:01 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
12126	11984	0	84	20	38

⚪ 2025-03-06 12:40:24 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-03-06 12:55:55 UTC Some tests failed, follow the links below. This fail is not in blocking policy yet Going to retry failed tests...

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
199 (only retried tests)	158	0	7	1	33

⚪ 2025-03-06 12:56:07 UTC ya make is running... (failed tests rerun, try 3)
🟢 2025-03-06 13:08:09 UTC Tests successful.

Test history | Ya make output | Test bloat | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
71 (only retried tests)	39	0	0	1	31

🟢 2025-03-06 13:08:16 UTC Build successful.
🟢 2025-03-06 13:08:45 UTC ydbd size 3.7 GiB changed* by +752 Bytes, which is < 100.0 KiB vs main: OK

ydbd size dash	main: cda52e1	merge: e126fe7	diff	diff %
ydbd size	3 995 175 008 Bytes	3 995 175 760 Bytes	+752 Bytes	+0.000%
ydbd stripped size	1 389 081 256 Bytes	1 389 081 640 Bytes	+384 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]

⚪ 2025-03-06 11:23:12 UTC Pre-commit check linux-x86_64-relwithdebinfo for e126fe7 has started.
⚪ 2025-03-06 11:23:29 UTC Artifacts will be uploaded here
⚪ 2025-03-06 11:26:29 UTC ya make is running...
🟡 2025-03-06 12:26:38 UTC Some tests failed, follow the links below. Going to retry failed tests...

Test history | Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
26491	23907	0	1	2467	116

⚪ 2025-03-06 12:29:24 UTC ya make is running... (failed tests rerun, try 2)
🟢 2025-03-06 12:41:27 UTC Tests successful.

Test history | Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
162 (only retried tests)	58	0	0	0	104

🟢 2025-03-06 12:41:37 UTC Build successful.
🟢 2025-03-06 12:41:56 UTC ydbd size 2.1 GiB changed* by -192 Bytes, which is <= 0 Bytes vs main: OK

ydbd size dash	main: cda52e1	merge: e126fe7	diff	diff %
ydbd size	2 293 954 392 Bytes	2 293 954 200 Bytes	-192 Bytes	-0.000%
ydbd stripped size	480 563 328 Bytes	480 563 264 Bytes	-64 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}