25-3: schemeshard: stats processing opt 1b

[ijon]

Cherry-pick from main:

• 626410b, schemeshard: stats processing optimizations #27165
• fe49740, schemeshard: fix stats processing opt 1 #28834
• 3950ae8, schemeshard: fix stats processing opt 1, part 2 #29664
• 585a708, schemeshard: fix stats processing op1, part 3 #29813

Streamline and optimize datashard statistics processing.

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]

⚪ 2025-12-01 15:39:22 UTC Pre-commit check linux-x86_64-release-asan for d44a576 has started.
⚪ 2025-12-01 15:40:01 UTC Artifacts will be uploaded here
⚪ 2025-12-01 15:44:21 UTC ya make is running...
🟡 2025-12-01 18:00:11 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?
15937	15277	0	321	323	16

🟢 2025-12-01 18:01:32 UTC Build successful.
🟢 2025-12-01 18:01:57 UTC ydbd size 3.8 GiB changed* by +15.5 KiB, which is < 100.0 KiB vs stable-25-3: OK

ydbd size dash	stable-25-3: eb91e15	merge: d44a576	diff	diff %
ydbd size	4 065 887 008 Bytes	4 065 902 832 Bytes	+15.5 KiB	+0.000%
ydbd stripped size	1 514 804 224 Bytes	1 514 817 600 Bytes	+13.1 KiB	+0.001%

^{*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]

🟢 2025-12-01 15:40:10 UTC The validation of the Pull Request description is successful.

[github-actions]

⚪ 2025-12-01 15:39:26 UTC Pre-commit check linux-x86_64-relwithdebinfo for d44a576 has started.
⚪ 2025-12-01 15:39:39 UTC Artifacts will be uploaded here
⚪ 2025-12-01 15:43:16 UTC ya make is running...
🟡 2025-12-01 17:16:10 UTC Some tests failed, follow the links below. Going to retry failed tests...

Ya make output | Test bloat

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
38812	36134	0	7	2650	21

⚪ 2025-12-01 17:19:12 UTC ya make is running... (failed tests rerun, try 2)
🟡 2025-12-01 17:35:50 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?
566 (only retried tests)	550	0	1	0	15

⚪ 2025-12-01 17:36:00 UTC ya make is running... (failed tests rerun, try 3)
🔴 2025-12-01 17:55:06 UTC Some tests failed, follow the links below.

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

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
552 (only retried tests)	537	0	1	0	14

🟢 2025-12-01 17:55:14 UTC Build successful.
🟢 2025-12-01 17:55:36 UTC ydbd size 2.3 GiB changed* by +20.9 KiB, which is < 100.0 KiB vs stable-25-3: OK

ydbd size dash	stable-25-3: eb91e15	merge: d44a576	diff	diff %
ydbd size	2 442 813 944 Bytes	2 442 835 376 Bytes	+20.9 KiB	+0.001%
ydbd stripped size	522 835 976 Bytes	522 847 688 Bytes	+11.4 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}