KVStore: Refine parallel prehandle snapshot (part-3)

[JaySon-Huang]

What problem does this PR solve?

Issue Number: ref #8081

Problem Summary:
In the previous implementation, the code is kind of messy. We will create a SSTFilesToBlockInputStream, then resetSoftLimit a soft limit to the input stream.

What is changed and how it works?

• Introduce SnapshotSSTReader to simplify the code. The following methods moved from SSTFilesToBlockInputStream to the new class:
  • getApproxBytes
  • findSplitKeys
  • maybeSkipBySoftLimit
  • maybeStopBySoftLimit
  • checkFinishedState
• Create SnapshotSSTReader with given SoftLimit, then use the "SnapshotSSTReader with soft limit" to create "SSTFilesToBlockInputStream"

struct SnapshotSSTReader
{
    SnapshotSSTReader(
        const SSTViewVec & snaps,
        const TiFlashRaftProxyHelper * proxy_helper,
        RegionID region_id,
        UInt64 snapshot_index,
        const ImutRegionRangePtr & region_range,
        std::optional<SSTScanSoftLimit> && soft_limit,
        const String & log_prefix);

    // Currently it only takes effect if using tablet sst reader which is usually a raftstore v2 case.
    // Otherwise will return zero.
    size_t getApproxBytes() const;
    std::vector<std::string> findSplitKeys(size_t splits_count) const;

    using SSTReaderPtr = std::unique_ptr<SSTReader>;
    SSTReaderPtr write_cf_reader;
    SSTReaderPtr default_cf_reader;
    SSTReaderPtr lock_cf_reader;
};

Check List

Tests

• Unit test
• Integration test
• Manual test (add detailed scripts or steps below)
• No code

Side effects

• Performance regression: Consumes more CPU
• Performance regression: Consumes more Memory
• Breaking backward compatibility

Documentation

• Affects user behaviors
• Contains syntax changes
• Contains variable changes
• Contains experimental features
• Changes MySQL compatibility

Release note

None

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pull-integration-test	dc52cf9	link	true	/test pull-integration-test

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