Add MVCC support and integrate HLC timestamps

[bootjp]

No description provided.

[gemini-code-assist]

Summary of Changes

Hello @bootjp, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request fundamentally upgrades the key-value store's concurrency and consistency model by introducing Multi-Version Concurrency Control (MVCC) and integrating Hybrid Logical Clocks (HLC). The MVCC store allows for concurrent reads and writes without blocking, maintaining multiple versions of data, while HLC provides a mechanism for generating globally consistent, monotonically increasing timestamps across distributed nodes. This change streamlines transaction management within the Raft FSM, moving from a lock-based approach to a timestamp-based optimistic concurrency control, and sets the foundation for more robust distributed data management.

Highlights

• MVCC Storage Engine: Introduced a new "mvccStore" in the "store/" package, replacing the previous simple memory store and separate TTL store, enabling multi-version concurrency control.
• Hybrid Logical Clocks (HLC): Implemented "HLC" in "kv/hlc.go" to provide monotonically increasing timestamps, crucial for MVCC and distributed consistency.
• Timestamp Integration: HLC timestamps are now integrated throughout the system, including the "kv/coordinator.go" for issuing transaction "StartTS" and "adapter/internal.go" for forwarding requests.
• FSM Refactoring: The "kv/fsm.go" has been significantly refactored to leverage the new "MVCCStore" and apply mutations with HLC timestamps, simplifying transaction handling by removing explicit lock management.
• Updated Infrastructure: Various components like "cmd/server/demo.go", "adapter/test_util.go", "kv/shard_router_test.go", "kv/snapshot_test.go", and "main.go" have been updated to instantiate and utilize the new "MVCCStore" and HLC.
• New Documentation: A new "AGENTS.md" file has been added, outlining repository guidelines, including project structure, build commands, coding style, and testing guidelines, with references to MVCC and HLC.

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[gemini-code-assist]

Code Review

This pull request introduces significant changes by adding an MVCC storage layer and integrating Hybrid Logical Clocks (HLC) for timestamping, moving away from a 2PC locking model. The changes are extensive and touch core parts of the storage and transaction handling logic. While the HLC implementation and the overall refactoring to support MVCC are well-structured, I've identified a few critical correctness issues that need to be addressed.
Specifically, there's an issue in transaction timestamping for forwarded requests that could break atomicity. More critically, the new MVCC implementation is missing the conflict detection logic required for Optimistic Concurrency Control, which could lead to lost updates. There is also a sorting bug in transactional scans. Please see the detailed comments for each issue.