KVStore: Refine parallel prehandle snapshot (part-1)

[JaySon-Huang]

What problem does this PR solve?

Issue Number: ref #8081

Problem Summary:

I'd like to introduce a new layer SnapshotSSTReader and move getApproxBytes/findSplitKeys into that layer. This could make the logic in parallel prehandle snapshots more clear.

This PR is a preceding PR that refactors the code to simplify the following logical change.

New SnapshotSSTReader interface in later PR:

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;
};

What is changed and how it works?

• Unify extra_id by split_id
• Unify the return type of executeParallelTransform and executeTransform to be std::tuple<ReadFromStreamResult, PrehandleResult>
• Apply "early return" on executeParallelTransform to simplify the code indent layer

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

[Lloyd-Pottiger]

Others LGTM

[CalvinNeo]

lgtm

[ti-chi-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: CalvinNeo, Lloyd-Pottiger

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[ti-chi-bot]

[LGTM Timeline notifier]

Timeline:

• 2024-07-05 09:33:36.141627779 +0000 UTC m=+4513.376861891: ☑️ agreed by Lloyd-Pottiger.
• 2024-07-05 11:27:00.459107998 +0000 UTC m=+11317.694342108: ☑️ agreed by CalvinNeo.