refactor(raft): migrate from openraft to raft-rs

[slow2342]

Summary

Replace openraft with raft-rs (TiKV fork) for Raft consensus. The new architecture uses a dedicated thread event loop with crossbeam channels, a RocksDB-backed log store, and a RaftHandle trait that abstracts the raft implementation from the API layer.

Use git dependency pinned to ad13f3d9 to fix RUSTSEC-2024-0437 (protobuf v2 stack overflow vulnerability).

Changes

• src/raft/node.rs — Raft event loop on dedicated thread with tick-based progression. Bootstrap saves initial ConfState before creating RawNode to fix single-node cluster startup.
• src/raft/raftrs_store.rs — RocksDB-backed raft log and state storage implementing raft-rs Storage trait.
• src/raft/raftrs_handle.rs — RaftHandle impl communicating with event loop via channels.
• src/raft/handle.rs — Trait abstraction (RaftHandle) for KvService/ClusterService.
• src/raft/mod.rs — Updated RaftRequest/RaftResponse types, simplified proto to opaque byte payloads.
• src/raft/network.rs — Updated network sender for raft-rs message types.
• src/raft/rpc.rs — Updated RPC server for raft-rs.
• src/raft/state_machine.rs — Apply log entries to storage engine.
• src/api/cluster.rs — Updated cluster service for raft-rs handle.
• src/api/kv.rs — Updated KV service for raft-rs handle.
• src/main.rs — Bootstrap and initialization changes.
• Cargo.toml — Switch raft from crates.io to git dependency (fixes RUSTSEC-2024-0437).
• proto/raft.proto — Simplified to opaque byte payloads.

Testing

• cargo build succeeds
• cargo test passes (10 tests)
• cargo run boots single-node cluster successfully
• ConfState bootstrap fix verified — no more panic on empty store

How to verify

cargo build
cargo test
cargo run  # should start without panic