schemeshard: preserialize Table.SplitBoundary for describe result

[ijon]

Preserialize table's split boundaries the same way table partitions are. The size of both depend on the same variable: number of shards in the table, but TablePartitions was preserialized (and cached) while Table.SplitBoundaries wasn't. Preserializing all potentially huge parts of DescribeSchemeResult message before it gets to the interconnect saves interconnect actors additional serialization costs. And when partitioning of the huge tables goes through the period of a rapid change that additional serialization causes interconnect to overload.

Single shortcoming though: preserialized SplitBoundary is not used (cannot be used) when boundaries of the index tables are requested through describe request on table index.

#6673
KIKIMR-21686

Changelog category

• Not for changelog

[github-actions]

⚪ 2024-07-16 09:43:43 UTC Pre-commit check for 78bb215 has started.
⚪ 2024-07-16 09:46:59 UTC Build+Tests linux-x86_64-release-clang14 is running...
🟢 2024-07-16 09:54:34 UTC Build successful.

[github-actions]

⚪ 2024-07-16 09:49:50 UTC Pre-commit check for 78bb215 has started.
⚪ 2024-07-16 09:52:31 UTC Build+Tests linux-x86_64-relwithdebinfo is running...
🟢 2024-07-16 11:43:22 UTC Build successful.
🔴 2024-07-16 11:51:06 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
38204	32960	0	5	5221	18

🟢 2024-07-16 11:51:50 UTC ydbd size 8.0 GiB changed* by +37.4 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 457b2b4	merge: 78bb215	diff	diff %
ydbd size	8 642 821 000 Bytes	8 642 859 264 Bytes	+37.4 KiB	+0.000%
ydbd stripped size	470 528 448 Bytes	470 530 112 Bytes	+1.6 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]

⚪ 2024-07-16 09:55:35 UTC Pre-commit check for 78bb215 has started.
⚪ 2024-07-16 09:58:53 UTC Build+Tests linux-x86_64-release-asan is running...
🟢 2024-07-16 12:07:09 UTC Build successful.
🔴 2024-07-16 12:09:08 UTC Some tests failed, follow the links below.

Test history | Test log

TESTS	PASSED	ERRORS	FAILED	SKIPPED	MUTED?
9696	9111	0	82	325	178

🟢 2024-07-16 12:09:50 UTC ydbd size 5.2 GiB changed* by +85.8 KiB, which is < 100.0 KiB vs main: OK

ydbd size dash	main: 457b2b4	merge: 78bb215	diff	diff %
ydbd size	5 548 952 816 Bytes	5 549 040 696 Bytes	+85.8 KiB	+0.002%
ydbd stripped size	1 194 957 904 Bytes	1 194 961 424 Bytes	+3.4 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}