[refactor] Refactor kv rpc dispatch

[ji-huazhong]

Background

There is a substantial amount of duplicated implementation logic between the current KV interface and Controller‑side RPC:

• Identical business logic is maintained separately for the synchronous and asynchronous KV APIs.
• The Client re‑implements Controller request construction, transmission, reception and response validation repeatedly.
• The Controller relies on lengthy if/elif branches to dispatch different requests, resulting in high maintenance overhead.

Changes Made

• Make the asynchronous KV API the sole high‑level implementation. Synchronous APIs invoke their asynchronous counterparts via the client’s background event‑loop.
• Add a unified Controller RPC request method to centralise message sending, reception, logging and response type validation.
• Refactor Controller request dispatching to use explicit handler mapping.
• Extract shared response‑building logic and consolidate performance metrics collection within the dispatch layer.
• Approximately 305 lines of redundant code have been removed in net terms.

Compatibility

This commit only refactors internal implementations and introduces no alterations to:

• Public synchronous and asynchronous APIs
• ZMQ request and response types
• Response data schemas
• Controller metric names
• Storage‑backend and distributed‑system behaviour