Fix manul failover with CLIENT PAUSE/UNPAUSE

[Paragrf]

Motivation

Implement Controller modifications to ensure master-replica data consistency during failover, aligning with server-side changes in apache/kvrocks#3377

Solution

Step 1 (Pause): Send CLIENT PAUSE WRITE from the controller to the current master.

Step 2 (Wait): Monitor the master-replica sequence gap until it hits zero, ensuring no data loss.

Step 3 (Metadata): Update the global topology metadata for the switchover.

Step 4 (Switch & Unpause): Promote the target and demote the old master; then explicitly call CLIENT UNPAUSE on the old master to restore its status.

Step 5 (Replicate): Reconfigure all other followers to sync from the new master.

Configuration Options

To prevent excessive blocking durations during periods of high write traffic, a maximum pause timeout has been introduced; the failover process will fail if the synchronization times out. The following parameters are added to the Controller failover configuration:

• "force_on_timeout": false,

• "sync_timeout_ms": 100,

• "pause_timeout_ms": 500

Related Issues

Fixes #384

[Paragrf]

In actual testing with the Controller and Kvrocks deployed in the same IDC, the system achieved a single-node write QPS of 10k/s with 20MB/s throughput. The write-stop duration (stall time) remained consistently under 10ms.