flushWatcher loop may call refreshPluginState frequently when watch chan closed

## Bug Report

Please answer these questions before submitting your issue. Thanks!

### 1. Minimal reproduce step (Required)

https://github.com/pingcap/tidb/blob/90e272abc31d22024c36ab095544bf20ba7f3575/pkg/plugin/plugin.go#L279-L289

This above code does not handle case `watchChan` closed, so `refreshPluginState` will be called frequently when this happens.

### 2. What did you expect to see? (Required)

When `watchChan` closed , we should close watch chan

### 3. What did you see instead (Required)

closed chan is not handled correctly

### 4. What is your TiDB version? (Required)

	func (w *flushWatcher) watchLoop() {
	watchChan := w.etcd.Watch(w.ctx, w.path)
	for {
	select {
	case <-w.ctx.Done():
	return
	case <-watchChan:
	_ = w.refreshPluginState()
	}
	}
	}

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flushWatcher loop may call refreshPluginState frequently when watch chan closed #49273

Bug Report

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

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flushWatcher loop may call refreshPluginState frequently when watch chan closed #49273

Description

Bug Report

1. Minimal reproduce step (Required)

2. What did you expect to see? (Required)

3. What did you see instead (Required)

4. What is your TiDB version? (Required)

Metadata

Metadata

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Labels

Type

Projects

Milestone

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