Implement Multi-node Quorum-based Health Checking for Kvrocks Nodes

**Motivation**
In a multi-controller deployment, triggering failover based strictly on a single controller's local probes is risky. If that controller experiences partial network degradation to a Kvrocks node — while its peers observe a healthy state — it may initiate an unwarranted failover, leading to a split-brain scenario and unnecessary traffic shedding.

**Implementation**

- Voting mechanism: Each controller independently probes every Kvrocks node. Before promoting a new master, the leader collects **unanimous votes** from all peer controllers via `POST /internal/vote`. A single NO (or unreachable peer with a live lease) blocks the failover.

- Peer discovery: Controllers register themselves in the store with a heartbeat. `ListActivePeers` returns peers whose leases are still alive. Expired peers are excluded from quorum rather than blocking failover.