fix deadlock while drainer.Syncer panic (#806)

[sre-bot]

cherry-pick #806 to release-3.0

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What problem does this PR solve?

we start Syncer goroutine in drainer

s.tg.GoNoPanic("syncer", func() {
	defer func() { go s.Close() }()
	if err := s.syncer.Start(); err != nil {
		log.Error("syncer exited abnormal", zap.Error(err))
	}
})

if s.syncer panic (like kafka producer panic), s.syncer.Start() would exit directly and the logics will happen

• drainer calls s.syncer.Close() in s.Close()
• s.syncer.Close() would wait <- s.syncer.closed take effetcs,
• but s.syncer.closed only can be closed in s.syncer.run()

What is changed and how it works?

add defer close(s.syncer.closed) in s.syncer.run()

Check List

[july2993]

LGTM