sync etcd endpoints immediately after initializing the client

[vldmit]

Problem Statement

etcd client would only run Sync after AutoSyncInterval (30 seconds), which makes tikv client vulnerable to the failure of endpoints provided in addrs before the first sync happens. Specific failure scenario:

1. tidb is initialized with a --path=endpoint
2. tidb successfully established connection to the endpoint
3. n < 30 seconds after, endpoint fails (e.g. k8s control plane is upgrading the pod)
4. etcd client is no longer connected
5. Safe checkpoint expires and CheckVisibility in KVStore start to error out

Fix

We explicitly synchronize client endpoints with endpoints from etcd membership during client initialization phase. etcd client would continue to do periodic sync, we just force first sync to happen in the init phase.

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[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: zyguan

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[LGTM Timeline notifier]

Timeline:

• 2025-02-14 01:27:25.878162368 +0000 UTC m=+579088.274384427: ☑️ agreed by zyguan.