schedule: if aba modification occurs, split-merge-interval will not take effect

[lhy1024]

Bug Report

What did you do?

test merge and split region

1. start a cluster while split-merge-interval is 1s
2. merge empty regions
3. set split-merge-interval to 1h
4. split some regions
5. again set split-merge-interval to 1s

What did you expect to see?

merge empty region

What did you see instead?

What version of PD are you using (pd-server -V)?

[lhy1024]

Because we first check whether recently started rather update config, which is introduced after #4616

pd/pkg/schedule/checker/merge_checker.go

Lines 91 to 107 in cbba49a

	expireTime := m.startTime.Add(m.conf.GetSplitMergeInterval())
	if time.Now().Before(expireTime) {
	mergeCheckerRecentlyStartCounter.Inc()
	return nil
	}
	m.splitCache.UpdateTTL(m.conf.GetSplitMergeInterval())
	if m.splitCache.Exists(region.GetID()) {
	mergeCheckerRecentlySplitCounter.Inc()
	return nil
	}
	// when pd just started, it will load region meta from region storage,
	if region.GetLeader() == nil {
	mergeCheckerNoLeaderCounter.Inc()
	return nil
	}

step	splitCache.ttl	region.ttl
start a cluster while split-merge-interval is 1s	1s	-
set split-merge-interval to 1h	1s, because recently start, no update	-
split some regions	1s	1h
again set split-merge-interval to 1s	1s, and no update	1h