schemeshard: preserialize Table.SplitBoundary for describe result (#6648)

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.

KIKIMR-21686

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