schemeshard: stats processing optimizations

[ijon]

Streamline and optimize datashard statistics processing.

Changes are mostly independent. Better reviewed commit-by-commit.

Profile guided optimizations of PersistSingleStats() (in synthetic test).
Total gain is around 30%.

• remove unreasonable iteration over entire ShardInfos
• single Now() timestamp for entire stats batch
• optimize number of lookups
• stop building now unnecessary storage pool kind mappings
• remove table/store aggregated stats copying
• collect ExternalBlobsEnabled only on PartitionConfig change
• replace ETxType->CounterId map with absl::flat_hash_map
• remove extra OpType->TxType lookup
• remove call to GetMainTableForIndex for not-index-table shards

[github-actions]

🟢 2025-10-23 19:57:54 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-10-23 13:20:42 UTC Pre-commit check linux-x86_64-release-asan for 69bc8af has started.
⚪ 2025-10-23 13:20:57 UTC Artifacts will be uploaded here
⚪ 2025-10-23 13:24:34 UTC ya make is running...
🟡 2025-10-23 15:33:01 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?
15958	15431	0	254	258	15

🟢 2025-10-23 15:33:07 UTC Build successful.
🟢 2025-10-23 15:33:28 UTC ydbd size 3.8 GiB changed* by +28.5 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 74cf4c1	merge: 69bc8af	diff	diff %
ydbd size	4 053 820 208 Bytes	4 053 849 424 Bytes	+28.5 KiB	+0.001%
ydbd stripped size	1 505 358 464 Bytes	1 505 382 464 Bytes	+23.4 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}

[github-actions]

⚪ 2025-10-23 13:20:43 UTC Pre-commit check linux-x86_64-relwithdebinfo for 69bc8af has started.
⚪ 2025-10-23 13:20:57 UTC Artifacts will be uploaded here
⚪ 2025-10-23 13:24:35 UTC ya make is running...
🟡 2025-10-23 15:00:10 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
39215	36429	0	6	2756	24

⚪ 2025-10-23 15:00:21 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-10-23 15:20:38 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
8106 (only retried tests)	8081	0	1	0	24

⚪ 2025-10-23 15:20:43 UTC ya make is running... (failed tests rerun, try 3)
🟢 2025-10-23 15:39:32 UTC Tests successful.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
439 (only retried tests)	420	0	0	0	19

🟢 2025-10-23 15:39:35 UTC Build successful.
🟢 2025-10-23 15:39:53 UTC ydbd size 2.3 GiB changed* by +22.8 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 74cf4c1	merge: 69bc8af	diff	diff %
ydbd size	2 436 392 128 Bytes	2 436 415 504 Bytes	+22.8 KiB	+0.001%
ydbd stripped size	517 948 936 Bytes	517 961 480 Bytes	+12.2 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}