feat(split-brain): Introduce lease mechanism for Kvrocks master

[redwood9]

Based on the ideas I proposed in issue #3380
#3380

this is the first step of the solution:
introducing a lease mechanism for the Kvrocks master node.
This mechanism ensures that the maximum split-brain window is bounded by the lease TTL. It also comes with flexible configuration options:

1. No config / disabled — Behaves exactly like the current system. If the lease feature is not enabled, there is zero impact on existing behavior.
2. Reject writes — When the lease expires, the node rejects all write operations to prevent any data conflicts / divergence.
3. Log only — On lease expiration, the node only logs an error (no write rejection). This makes it easy to catch via monitoring / alerting systems without disrupting traffic.

To minimize invasion into the existing codebase structure, the implementation follows these strategies:

1. Controller probing — Add a new HEARTBEAT command (non-conflicting with existing standard Redis commands). The response format remains unchanged to keep controller integration simple and backward-compatible.
2. Unified lease check — Add the lease validation logic inside Storage::writeToDB(), so all write paths are guarded in one central place.

I will also submit the corresponding HEARTBEAT implementation in the controller at the same time / in parallel.